As everyone probably knows, you take certain risks when purchasing a used chainsaw sight unseen, especially when it may have been used and abused by a commercial outfit of some sort.  How will you know if it runs or not, and how much life is left in it?  Here are some things you can do/check to decrease the chances you get a saw that's all jacked up.

• Ask for a compression reading on the saw (should be about 150 PSI), or ask for a guarantee of compression over a certain number, say 140 [you can test this easily with a $12 tester from your local auto parts store - or better yet, a used one bought on e-bay].  I'm amazed that some sellers tout themselves as mechanics and have photos of their saws in workshops, but when you request a compression test, they say "sorry, I don't have one".  Beware.
• Ask if the piston or cylinder is scored (this can be determined by removing the muffler and inspecting them visually on many saw models).
• Ask for close-up photos of all sides of the saw, especially of the BOTTOM of the saw where wear is most likely to be evident (from repeatedly sitting it down on hard surfaces during multiple jobs or bouncing around on the back of a trailer, or just plain thrown around alot). 
• A saw with a replacement bar or one with most of the paint missing is not compatible with a "lightly used" saw.  I have a Stihl 029 Farm Boss that I bought in 2000 and truly used it lightly during this time and it has almost all of the paint on the original bar.  If you've worn out the bar on a saw, you've run more than a few tanks through it.
• Ask what the saw was used for (Commercial operations?  Cutting stumps?  These are high wear/abuse activities).
• Ask if the seller is the original owner of the saw and why they're selling it.
• Ask for sales receipts if available so you know the date of purchase. 
• Ask yourself why a "lightly used" saw has ANY cracks in the plastic or broken parts - regardless if they are "only cosmetic and don't affect the function of the saw", you should wonder if such cracks are compatible with a well-cared -for saw.  Ask yourself why a saw or bar might have been repainted.
• Ask the seller if the chain brake, choke (if applicable), and oiler work.
• Ask if the saw starts easily and idles properly.
• If you're buying a vintage or used saw, and if it is commanding a pretty penny (like an 070 or 090), ask for a video of the saw starting, idling, and even cutting.  Some of the recent 090s sold on e-bay had such videos on youtube and commanded over $1500.  This is good advice for sellers too - if you want top dollar for your saw, you're going to have to put a little effort into advertising it and providing the bidders with helpful information.  One seller even posted a video of him testing the compression of the saw!

An honest seller should have no problem telling you all about where he got the saw and why he's selling it and in providing the above information at least for high dollar saws.  All of the sellers from whom I bought saws were very open and forthright about their saws and very happy to satisfy my informational needs about the saw.

One solution to this whole problem of the used saw may be to buy various "new", "in box" or "never fueled" Stihl chainsaws that can be found listed on e-bay.  But these saws are of questionable provenance.  Stihl does not permit their saw prices to be advertised on the internet, and more importantly, they ONLY allow their new saws to be sold by authorized dealers.  Moreover, you'll get no warranty if it's not purchased from the dealer.  So, one has to question how an e-bay seller may have got a hold of a "new" saw.  If you have ever purchased a new Stihl saw from a dealer, you know that they fuel and test run the saw prior to its leaving the shop and they are almost always sold with a bar and chain.

So, where do these saws come from?  There are four (or more) possibilities:

1.)  The seller is an individual who bought the saw from an Authorized dealer, (and maybe it was indeed not fueled prior to the sale), and the seller just decided s/he didn't want/need the saw anymore so s/he's selling it on ebay.  (Maybe they're like me and they bought an 880 and then just realized that it's too ridiculously big for an occasional user.)  These saws are rarely/never put "on sale" by the dealerships so it's very unlikely that the seller got it for a "clearance price" and put it on ebay.  Stihl apparently guards manufacture dates quite tightly so that customers won't bully the salesforce asking for discounts on "last year's model".  This is part of the reason they're rarely put on sale, as dealerships don't have to "clear out" last year's models, unless a significant model change occurs, which is infrequent and last happened in about 2002 when the emissions requirements changed.  That's when, for example, the 029 Super became the MS 290.

2.)  An authorized dealer is the seller of the saw on e-bay or s/he uses an accomplice to sell it, in order to thin out his/her inventory or for any other reason.  Or, the seller "bought out" the inventory of an authorized dealer who was going out of business or something.  I purchased a Sthil Splitting Maul from such a seller.  I heard another person speculate that the dealers get more money for their saws on e-bay due to "bidding frenzies" but I find this suspect.

3.) The saw was STOLEN from an authorized dealer.

4.) The Stihl saw was made in Brazil NOT Germany.  I don't think these saws are even regularly imported to the USA, but to be honest I'm not exactly sure what the importation requirements are or where they come from.  Please e-mail me if you know more about this.  I have seen a number of saws on e-bay that were made in Brazil not germany. A German saw of the new MS series should have a paper sticker that clearly reads "Made in Germany" on the front of the saw.  Many older saws say "Made in Germany" on the starter side under the Stihl logo.  It is also stamped on the bar near the bumper spikes, and painted on the bar (if the saw comes with a bar and chain - many of the "new" saws on e-bay do not, for reasons that are not clear to me).  All six newer model Stihl saws I personally own also have an Underwriter's Laboratories sticker on them, which was not visible on the Brazilian saws I have seen photos of.  Also, the Brazilian saws I have seen have bumper spikes that are steel "in the white", while all German saws I have seen have bumper spikes that are gold-colored like a grade 70 transport chain. The proof of where it was made is in seeing the "Made in Germany" markings, or confirming the serial number.

I have been following the sales of Stihl professional saws on e-bay for 2 years now.  My observation is that you save very little buying a new or like new saw on e-bay.  Tax and shipping cancel themselves out, and most very nice saws on e-bay command very near the retail price.  To get $20 or $40 in savings you lose your warranty, and don't have a relationship with your dealer to help you if something goes wrong.  In my opinion, it's not worth it unless you're buying a large high-end saw where the savings will be higher.  On my 660 and 880, I estimated that in the end I saved a net 15% on these saws after accounting for tax and shipping, etc.  So that's a few hundred on the 880 and a couple hundred saved on the 660.  For the smaller saws, you're probably better off just getting a new one from the dealer, as I have done.

All of this notwithstanding, I did recently complete the purchase of my seventh Stihl saw on e-bay (I have a 200T, 260, 029, 361, 460, 660, 880, and two 090AVs). The 660, 880, and 090AV were e-bay purchases, and I have no complaints about them.  I also bought and then later sold another 090 on e-bay.  All of these saws have good/high compression.

Good luck.  And always remember, "Trust, but verify."

Many folks have been asking me lately how big of a saw/bar to get.  I can comment only on Stihl saws, since I don't have experience with other saws.

In general, my advice is this:  Get the biggest saw you can afford and handle comfortably, with the smallest bar you need for the majority of your jobs.  Note the emphasis on "handle comfortably" and "afford" and "majority of your jobs".

If we were all wealthy and burly lumberjacks with meathooks the size of catchers' mits, and lots of experience running saws, then we would all run Sthil MS660s (or larger saws) for all but limbing and small tasks.  The problem with larger saws is that they're heavier, eat more fuel, and are more expensive and more dangerous because of their awesome power and the fact that generally you run high-kickback (yellow label) chain on pro saws.

Be honest with yourself.  If you're an occasional user and you rarely tackle a tree larger than 12 inches, you don't need anything larger than an MS 210 with a 14-16 inch bar.  That bar will allow you to cut a 24" tree should you ever need to.  (Go to youtube to see a guy cutting down a 40" silver maple with a Stihl 029 and 16" bar.)  You just make one cut on one side and then another on the other side.  And that shorter bar will be safer, lighter, and will allow the smaller saw to run better because it's not pulling the weight of a long chain against the friction generated by a long bar.  Much more important than having a big saw, you need to keep your chain sharp.  You will be better off with a smaller saw and 3-4 chains on hand than with a bigger saw and one chain.  Once 3 are dull, you install the fourth and take the 3 dull ones to the dealer for professional sharpening.  By the time you dull #4, the other three will be ready for you to pick up.  You will know that they need sharpening because instead of large wood CHIPS being discharged from the saw, a finer sawDUST will be (remember CHIPS not DUST).  Don't forget to turn over your bar (yes, so that STHIL is upside down every other time) each time you change your chain.  Also, if you have a choice between .325"/.063 and 3/8"/.050 chain when you buy a smaller saw, go with the former - it makes a narrower "kerf" cut, so your saw has to eat through less wood to make the cut and hence will cut faster.  (You have such a choice, for example, when you but a Stihl 290 "Farm Boss".  If your dealer doesn't have it set up with a .325 sprocket, you can ask him to change it out so that it will run the smaller chain.  The sprocket costs under $10 if he has to order it, and he should have the .325 bars and chains on hand.)

So, when do you need a big/ger saw?  If you're really big/strong, and/or you're cutting a lot of wood (5+ cords per year, clearing acres, etc.), cutting hard wood (pin oak, locust, etc.), or cutting big wood (24" diameter trees or greater) go for it.  Buy the biggest saw you can afford, and put on the smallest bar you need.  If your small/ish bar doesn't cut it (literally!) one day, you can just go get a bigger bar/chain combo for the job.  Meanwhile with the big saw/small bar/chain combo, you'll be just tearing through wood, eating it alive.  A Stihl MS 660 with a 20" bar is just awesome, you wouldn't believe how the chips just fly.  Again, if you're only cutting 2 hours per year, the added speed of the 660 is not worth the money and the sore back - but if you're cutting 10+ cords of wood over many many hours, it may well be worth it.

And always remember - the chainsaw is the most dangerous power tool there is (interestingly, the axe is the most dangerous hand tool).  Read the instruction and safety manual.  There is a lot more to know about using a chainsaw than just starting it and laying it on wood - and it's not all intuitive.  Don't learn the hard way, via injury.  Understand kickback, don't cut with the tip, beware of drop-through cut injury, don't cut overhead or from ladders, and wear eye and ear protection and chainsaw chaps, and preferably a helmet protection apparatus.  Chainsaw injuries are REAL, serious, and COSTLY, and they're going to happen to YOU, unless you are experienced, or you have familiarized yourself with proper saw operation.

Good luck.  Be safe, and happy cutting!