I would have to say that bad ebay experiences are rare but they happen.. I believe that 99% of people on ebay try to be honest. If someone has a lot of bad marks that is the tell tale sign not to deal with them.
I recently had a guy bid on a set of New Carbon Bontrager XXX Team Issue wheels and then at the last minute was outbid by a single feedback user. It turns out the person who out bid him was his wife. He basically shill bid himself and then stiffed me on buying the wheels.. He didn't want his feedback record tarnished so he used his wifes ebay id so he wouldn't have to pay. I had to re-list the wheels..