Like some of the other members have said. It is a hit or miss.
I have sold a lot of saws on eBay, and have bought quite a few also, but I am about out of it.
You would need to read the description carefully, look at the sellers feedback, and see if he has sold chain saws before.
There are not always bargains, I have seen saws bid up above retail, especially Stihl.
eBay sellers are like a lot of dealers in saws , some are very good, while some are something else.
eBay now has buyer protection, so if you get a saw that does not run, or is not as described, they will get your money refunded including shipping.
With all that said if you are buying a new saw, buy it from a local dealer, you will be ahead of the game in a lot of ways.