I have over two hundred unique positive feedbacks on eBay, but I have had some bad experiences. Even if you are very careful you will sooner or later recieve gear that is not as described. At least that has happened to me.
It's not necessarily that the seller is even trying to scam you, but there are some eBay "traders" that buy gear on eBay for cheap and then flip it for a fast profit. They haven't used the gear, don't know the gear, and probably haven't even tested it properly. And then there are some that just do pawn off bad gear.
One thing I do is check a sellers' last month of transactions. That way I can see if they recently bought the item they are now selling. It is not uncommon to indeed find that they just bought it and are turning it around quickly. That menas either that they discovered it is not working right, or they are just trying to make a fast profit.
The other main thing I do is communicate with the seller af the item. I ask questions about the condition of the gear, and if they are the original owner, how long have they used it, that sort of thing. I can tell a lot by their response to my questions, how quickly they respond, and if they even respond at all.
The suggestions by Nosferous are excellent, especially the part about the picture of the item for sale. However, you have to be careful about that. Make sure you look at other auction for the same item, especially ones that are over. Sometimes sellers "borrow" pictures from ended auctions and put them in their own auctions as if they are pictures of their item. I always take my own pictures, and have seen others use them in their auction, claiming they are pictures of the actual item. When I find that happening I always email and demand they remove the pictures.
All said, I have found my eBay suying and selling experiences to be generally very positive. The big thing is that I've been able to upgrade my studio for far less than I would have otherwise. I've been able to easily sell gear, and then put that money toward new gear. Also, There's some vintage pieces I've bought on eBay that I'm very glad to have, and that would have cost me *far* more in a used gear store. So overall, a good experience.