If you've been looking around for a new Xbox 360 from Microsoft, you'll probably have found that they are sold out at most stores. Bidding wars have started on eBay, with some systems going for almost twice the retail price.

Big retailers, including Best Buy Co. Inc. (BBY.N: Quote, Profile, Research) and Target Corp. (TGT.N: Quote, Profile, Research), reported running out of Microsoft's (MSFT.O: Quote, Profile, Research) next-generation game system at most, if not all, of their stores following its North American debut on Tuesday.

By Wednesday afternoon, 10,700 Xbox 360s had traded on eBay at an average price of $781, a spokesman for eBay said. The basic unit retails for $300. A premium package that includes accessories and a hard-drive that allows gamers to play existing Xbox titles costs $400.