I've bought quite a lot and sold a few items on ebay, most of it went very well but one in particular was a bit of a nightmare.
A new set of TaylorMade golf clubs from the US, which was less than half price in the shops over here - but they were a new range and not yet even on sale in the UK. Too good to be true - yup you bet they were.
After ages of trying to contact the seller in the US, I filed a complaint, which I had to chase up via Visa as Paypal were less than useless. I finally received a set of clubs but they didn't appear right, so I took them to my local golf pro shop. He instantly said they were fake and helped me get in touch with TaylorMade (TM). TM sent a courier to pick up the clubs and then sent me a letter saying they were fake and could I help with their investigations into fake imports.
The TM letter helped me to get a full and instant refund from Visa, whilst they began to try and recover the money from the US seller. A few days later, I received a phone call from a US attorney, who said he was working with the FBI and the seller I had bought the clubs from was already under investigation. I verified this seemingly far fetched story with TM, who confirmed that it was absolutely true.
I heped them with some new information, as I got the seller's mother's home address - something they had not had before. Under US law, your family (parents at least) are also liable for your debts.
To cut a long story short, the guy was caught and prosecuted by the FBI and his mother's house was repossesed. I got a brand new stand-bag from TaylorMade as a thankyou - at that time still not yet released over here and I bought a genuine set of clubs.
Other than that, I think I've been pretty lucky, but I never buy from someone with less that about 98% feedback and a couple of times I have asked previous buyers what the item/service was like, before I commit to buy.