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 090). In addition to the 660 and 880, I recently purchased a vintage 090 with good compression (175 PSI) and an unscored piston and it starts and runs beautifully.

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