Shill bidding is when someone ‘fake bids’ to artificially get the price up on an eBay auction. It can also happen when someone ‘fake buys’ an item to get a seller’s feedback up. If you watch prices and bidders carefully you can sometimes see the same bidder pushing prices up at the beginning of the auction and then disappearing. It is not allowed so if you do see it, report it to eBay.
Signs to look for are users who either do not follow through on opening bids of retract bids, are new, have no feedback and who always bid on a certain seller’s auctions. Naturally this can all happen ‘naturally’ but if it forms a pattern with certain buyers, then there is likely a shill bidder about.