When you place a bid, you are entering a legal contract to purchase an item at the price you have offered, . You can't enter a contract with yourself, so bids by yourself are void. It was a crime punished by execution in Rome (same as fractional reserve banking  ) Ebay is a good source for information regarding auction fraud actually. Bidding on your own auctions is called shill bidding, and is a felony in the US, and Europe.

Of course, until there is a court case that proves me otherwise, I'd say that auctions done informally in a thread on a forum are honor bound at best. I doubt someone here could successfully press charges against someone for shill bidding, however my point is that its not something you can just brush off as a no big deal type of thing.