I have had a couple of purchases go bad.. the sellers were both banned shortly AFTER I made a purchase.
The ebay resolution center is USELESS I have found Paypal to really work to get your money back, but I went to them too late the last time and so the last problem left me $100 in the hole..and I am very angry
I haven't had to do this myself, but it's what's recommended by many on photography forums; used photography equipment is easily several hundred to thousands of dollars.