Firstly, I will start by saying that the single biggest way to make a mistake is to rush a bid/purchase. This normally happens when the listing is closing soon and you only just found it. Or, you just decided you wanted something but haven't had time to research it. Try to avoid this situation by waiting until you've had time to do your homework and/or for another similar item to come along (and come along it will).

When buying a used lens there are several things you must know about the lens itself.

Does the lens fit your camera? To find this out, look in your camera manual to find out what lens mount your camera uses. Check the auction for information. Email the seller if necessary.

The lens fits, but will it work? Some older lenses will not work on newer digital cameras because they have a chip inside that is no compatible. Sometimes they can be 'rechipped', but normally this isn't econmoical in time or money and those lenses are simply no good. Search the forums on dpreview.com for information on any particular lens.

When choosing which of the hundreds of lenses to bid on, condition is paramount. No I'm not talking about how it looks. It's how it works that matters and there's a lot to consider. A lens that looks 'mint' might not work at all, and a lens that looks like a dogs dinner may work perfectly. Don't worry though, it's all detailed below. Follow these steps and checks to always get a good deal. Most importantly, make sure you can trust the seller. A battered lens from a good seller is often a better buy than a new one from a seller with worse feedback. The 'new' lens might be a return which already has problems.

Ideally the images and description on the auction will allow you to be certain about all the important areas. In reality there will probably be some omissions. If so, ASK the seller and get a straight answer. If you get an 'I don't know', or some other excuse assume the lens has a problem and just find another.

Here we go with the stuff you need to know:

Lens body: Any cracks/dings to the casing?

Iris/diaphragm: Is the aperture symetrical? If not, you will get weird bokkeh. Does it stop down correctly? A diaphragm that closes more/less than it should for any given f-stop selection or slower/faster than it should can cause under/over exposure of the image. If the diaphram is manual, does it work smoothly and quickly? If it sticks, that's a problem.

Optics: Any dust, fungus, scratches, separation, haze, coating breakdown, chips, marks or anything else inside or out? If so, get details and a STRAIGHT ANSWER. Can't overemphasise the straight answer thing. ;)

Focus. Is the MF smooth? Does the the AF work and is it accurate? Front/back focus is something most sellers know nothing about. - It's where the lens autofocusses consistantly in front or behind the subject due to miscalibration. It's expensive to fix and a lens with this problem can be useless unless used in manual focus. The problem is greater with longer focal length lenses and/or large apertures (small f-numbers) - where depth of field is shallower accentuating the problem. With a fish-eye lens, the problem will be hard to detect and is therefore less of an issue. Even with a fisheye lens, if you wanted to take a closeup of something in low light (open aperture), focus accuracy will be an issue. Many many lenses are sold because of focus issues. Often, the seller doesn't know about it themselves they just know the lens isn't as sharp as they'd like and sell it. Watch out for front/back focus as it is a common problem.

Lens mount: Is it damaged/worn? Is there any play when mounted on the camera?

Contacts: If the lens has electrical contacts (all AF lenses do), are they worn or corroded? If they're worn, it's a sign of very heavy use. If they're corroded, it could be a sign of water damage - possibly salt water - the ultimate enemy of electronics. In either case, find another lens. Another one will come along on eBay shortly.

Filter threads: Dings/scratches may make them useless. That means you can't mount your polarizer filter etc. A big ding to the filter ring might make mounting the hood impossible - a big problem when shooting in sunlight.

Tripod Mount. Should it come with one? Is it included? Are the threads intact?

Lens caps: If a lens doesn't come with caps, it's much more likely to be scratched. Are the caps original? If not, the lens spent some time without caps!

Case: A lens with a case and caps is likely to have been better protected and looked after than one without.

Hood: Did the lens originally come with a hood? Is it included? A hood is very desirable for any lens and is included with most originally.

Box: If you can get a lens with its original box, it's far more likely that you're buying from the original owner. It's less likely the lens has been around eBay 20 times with focus problems.

How old is the lens? An older lens will generally have received more use and therefore more knocks, the mechanics more wear etc etc. Dust and fungus spores will have had more opportunity to get into the lens. A very old lens that has no dust has probably been opened and cleaned. Maybe due to fungus. Ask about this before you buy and if you're not sure, avoid.

Has it received pro use? If so, was it in the field or the studio. If a lens has been used by a PJ, it will have had a beating. Probably in all weathers. The type of lens is often a clue to its history. A Canon 16-35mm f2.8 L is far more likely to have had pro field use than a kit lens. A special lens like the Canon 200mm f1.8 L or 400mm f2.8 L is almost guaranteed to have seen serious action.

Cosmetic marks. This is a final consideration to split equally priced lenses, but not strictly relevant. After all, you want the images to look good and if everything else is fine, the looks of the lens genuinely don't matter.

Other important things to think about:

Is the seller reputable? Have they sold similar items before? Have they got good feedback? Have they provided detailed images and description of all important parts of the lens (see above)?

Does the seller buy a lot of lenses listed for spares/repair or other lens/camera parts? If so, you may be buying a 'Frankenstein' lens rebuilt by an amateur.

Is the lens any good in the first place?! Is it the best value? What are the alternatives*? Check this by visiting sites like dpreview.com and reading about the lens. Ask what the alternatives are and read reviews. Check the new price of the lens at a good online retailer. sigma4less.com or b&hphotovideo.com in the US. warehousedirect.com in the UK. Read the MTF charts at the manufacturers' websites. Just Google "MTF chart" plus the name of the lens.

*I mentioned alternatives and it's worth considering whether there are any. For example, some people pay $3500 for a ten+ year old non-IS Canon 300mm f2.8L apparently because the new image stabilized model costs over $5000. However, have they considered that a brand new Sigma EX 120-300mm f2.8 is only $2080?! It is new (with no wear and full warranty) optically very close, more versatile - being a zoom etc. To some, the extra 50% for the Canon's fractionally better IQ will be worth the high risk of repair bills on that old and well used lens. To most, the Sigma would be the better choice until one could save up for the new 300mm f2.8 L IS.

Search pbase.com for images taken with the lens you are considering. These will generally be real world images and/or user reviews/comparisons rather than catalogue images taken specifically for selling the lens to the public.

REMEMBER that if you are buying from abroad, that on top of shipping, you will almost certainly have to pay, insurance, taxes and handling fees. These can combine to turn what seems like an eBay bargain into an unnecessary risk and quite possibly an expensive mistake. You may end up paying more for a lens with an international warranty (possibly meaning it has to be sent to Japan for service) than you would have paid for the same lens at your best national online retailer with a domestic warranty (meaning the lens can be serviced locally - faster and saves on shipping).

A note on feedback.

A seller with 98% positive feedback could be deliberately ripping off one in 50 people. Find a seller with the highest possible feedback and check their feedback comments. Do they regularly receive -ve? If so, you might well be the next person to have to leave -ve. Needless to say, if the seller only has feedback of 1 and 100% positive, this doesn't mean much. They might have bought a paperclip once. A seller with 1000 feedback and 99.5%+ positive with most of their feedback from buyers is a far better bet. You have a 4x greater chance of having a problem with a seller with 98%+ve than one with 99.5%+ve. Think about it. 

I am not saying that you should only buy from sellers with 100% positive feedback. If someone sells a lot of items, eventually after hundreds of deals, they are bound to get a problem with someone. It may just be that the buyer bid moer than they should have and tried to wriggle out of their commitment. Leaving -ve feedback when the seller rightly reported them to eBay for non-payment. So, just make sure a seller has a long track record selling the same sort of items you are interested in and that they don't regularly get -ve or neutral feedback.

How much to bid?:

You should check the previous sale prices of items available to your locality. You do this by using the advanced search - a very useful feature generally. Click on 'advanced search' under the search box. Next, in the advanced search page that comes up, type in what you're interested in searching for. Then near the top, check the "completed listings only" box. Then, scroll down and select the "items available to" field. Find your location in the drop-down and click search. You will see all the items that have recently been offered to your location. The final value will be in red if the item did not sell and in green if it did. Find both the lowest and highest values in green and compare these to what most of the items sold for. See if you can work out why the items sold for the prices they did. It will be a combination of many factors, condition, images, description, seller feedback, location and so on. Compare those listings to the one you're currently interested in to give you a guide on how much you should bid.

Personally, I always want to get the item for hardly any more than the lowest price it's ever sold for. That way, I know that if it's not quite as perfect as the seller says it is (and they almost always say it's mint), then even if it's not quite as good as I hoped, I still paid the minimum and so didn't lose anything. This way I don't win every auction, but I always get a good deal. Another identical lens - maybe better will alwasy come along soon.

I once paid a lot of money for a pro lens described as mint. It had all sorts of problems meaning that not only was it not worth what I paid, but I had to spend even more getting it repaired. Needless to say, that mistake wiped out the savings I made on many many other eBay deals! A lot of people use eBay to save money. So, no point spending more than you can get the same item for at the local retailer. Check this before you bid because sellers will alwasy claim the retail price is much higher than the best price you can find.

How many bargains do you have to get to make up for a $3000 mistake? What I mean is that , the higher the value of an item, the more cautious you have to be. If a seller won't answer questions or send additional pictures, just don't buy. Honest sellers will always provide a lot of info in the first place and will happily provide additional images etc.

Watch out for poor quality images! Buy from a seller whose images prove that they know their photography. That way, you know that the description was written by someone that knows what they're talking about. If you're buying a professional lens and the image on eBay is really poor quality, you have to ask yourself how much the seller knows about photography. If they can't take a photo, how do they know the detailed workings of a lens? How do they know there isn't anything wrong with it if they can't even operate a camera? Obviously you want to see the item close up. You don't want to see the seller's carpet! So a shot of a whole room with the item in the middle of the floor is no use. Get the seller to take a shot where the item fills the frame. Make sure it's in focus and there is no camera shake. If the seller can't manage this, they don't know what they're doing and you're best off finding one that does. Sometimes a seller may deliberately pretend to be unable to take a decent image to avoid showing up a problem with the item. Look at their other auctions. Are all the images that bad or just the ones he's showing you?

Sometimes images that look great are still poor quality, for the purposes of evaluating an item. I am talking about ultra-high contrast here. Sellers can make any item that contains mainly black or white/silver (almost all photographic gear) look as good as new by boosting contrast. This "clips" the details from the darkest/lightest areas and makes them look solidly one colour. This effectively hides dust inside lens optics, scratches and marks to the outside.

Make sure you have seen all the angles. If not, the angle you didn't see will be the one where the crack is! It only takes five shots to show every detail of a lens. The eBay picture pack allows six big images. That gives one extra to show any particular detail (damage etc). So, try buy from someone that is using the picture pack and has put six shots in there showing every angle and any important detail(s). Be sure that there is no filter on the lens in the images to hide marks on the front element. If there is a filter in the image, beware. Get the seller to send you an unedited close-up of the front element without the filter on there.

Ok, so you bought your lens and the package just arrived. How do you check it to make sure it's as it should be?

Checking your new lens:

Look for any obvious damage to the lens body or glass surfaces. Check everything listed in the auction is present. Caps, case, box, instructions, tripod mount etc. Now check the front and rear glass. Marks on the rear element are more serious than on the front and they are also more often over-looked. Check the mount for damage and that the contacts are clean.

Clean any dust off the external elements according to the manufacturer's instructions. Check the internal optics for dust/fungus etc etc by looking through the lens at a low power lamp from each end. You want the lamp just off center. DO NOT look at the sun to do this for obvious reasons! Move the lens around and open the lens aperture to look for dust etc. Minor dust is not a problem and is to be expected in any used lens. Even in some brand new ones. It's more significant in shorter focal lengths, so a fisheye with major dust could be a problem, whereas a 300mm+ lens with quite a bit of dust won't matter much. Fungus is a very serious problem that will grow worse with time. It will contaminate your other lenses and will etch the glass meaning that even cleaning it won't fix the problem. Even if it is cleaned off, some spores will remain and the fungus will return. Even a tiny hint of fungus is a serious problem. Get a refund. Google for images of lens fungus if you are not sure what to look for.

First take shots of a scene with a lens you know works. Take a shot wide open, f8 and f16 (you can do every aperture if you're keen). These are reference shots.

Now mount your new lens on your camera and check for any play in the mount. Does it fit like your other lenses? Now turn the camera on. Do all the controls work as normal?

Take similar shots of the same scene with your new lens. Compare exposures. Is the new lens over or under exposing relative to your reference shots?

Next select f8 and use your depth of field preview button. Look into the lens. Is the diapragm symetrical? Repeat at f22. If you don't have depth of field preview, just go to manual mode and set a long shutter speed. Look into the lens at the selected apertures and press the shutter release to view the diaphram stop down.

Now it's time to get serious.

Put the camere and lens on a tripod. Print a focus test chart (Google to find a free one) and read the (very quick) instructions on how to use it.

Take shots at all apertures to see if the lens focusses accurately. If not, it might be your camera. Repeat with your other lenses and see if they too have a similar problem, if so, it's likely to be your camera and not the lens. If not, it's probably the lens.

Up to you what to do if you have a problem with focus. If the seller told you there was no focus issue then you may wish to get a refund. If the lens is under warranty, you can probably get it serviced for free. Otherwise, it's an expensive job to get a lens recalibrated and it's best to send in your camera at the same time. Obviously if they recalibrate your camera, all your current lenses may not focus exactly as they did before...

Now stop measurebating and pixel peeping and get out there and take some real world images. Have a look at them and see if there's any problem. If not, hopefully you can pat yourself on the back for getting a perfect lens and saving hundreds after all taxes/fees/shipping/insurance/repairs versus the best price from your local online retailers.

If all is well, leave the seller some +ve feedback.

If there does turn out to be a problem, give the seller a chance to fix it and/or refund. Insist (politely and courteously) on a full refund including shipping, Paypal fees and whatever else you paid. It's not your fault the seller sent you cr@p!

If the seller won't make things right and fast, consider leaving them -ve feedback to warn others. If they solve the problem but slowly or not to your satisfaction, you might want to leave neutral feedback. If they solve the problem quickly and completely, you could still leave +ve for a problem well handled to let others know that even if there's a problem with an item from that seller, they don't have to worry.

One last tip. Do not rely on eBay or Paypal guarantees to protect you from being ripped off.

Hope all this helps.