There's also quite a good trade in Gmail invites, Trabant audio files, recipes for cookies, desktop wallpaper jpegs and other virtually worthless items. You can buy these things for a few cents and then the "seller" leaves you positive feedback for the transaction. If the seller sells several tens of thousands of these items, they're in the money. A con-artist buys them to build a 20 or 30 item positive feedback rating then offers brand new ipods (or similar) at very low prices. A victim comes along and sees the deal of the century from someone who has a big and all positive feedback rating and bids �40. They win the auction and their �40, along with ten other peoples' �40, heads off to Shanghai never to be seen again.
If possible, check what the person bought or sold to get their rating.