I've bought one or two car parts off eBay, and I always wonder how I would know whether the part is from a stolen car. (Answer: I really don't. And it'd be a great way for a chop shop to sell their parts. Most of them, I'm sure, are from legitimate junkyards, but there's no real way to tell.)
I've bought and sold off eBay, not a real fan in either respect. I'll buy cheap items that I don't care much about off eBay, but nothing serious like a lens or body. The way eBay handles feedback these days almost guarantees that problems will be hidden or at least reduced - it should be independent and hidden until both enter their responses; in reality the seller gets to review and respond to the buyer's feedback before entering his or her own, putting the buyer at risk of retribution for an otherwise honest review. (At least that's how it worked a couple years ago...it may have changed since then.)