It's a scam.
Usually, the scammer will send a check for a lot more than it's worth, and ask you to refund some money. After you refund some money, it turns out his original check or money order was counterfeit.
However, he could just be an honest bloke who really wants the item. If that's the case, he can follow the rules and bid on it like everyone else.
Yep, lots of scammers on ebay.