If you never mail back the excess, nobody is out anything. But these kind of transactions usually follow a series of email contacts, sometimes even telephone conversations, all designed to gain the "CON"fidence of the victim. The OP claims the excess money was called a "mistake." There are a multitude of other lies and stories, really a limitless number, that always end in the victim having to send back their own money. Sometimes they think they are getting to keep some of the "excess" for their troubles (using the victim's greed as the hook), but with auction sale scams they really need only prey on the victim's honesty. The victim's own valid funds clear the banking system and are gone in a few days, the bogus funds the victim received take a few weeks to be returned to the victims bank, and the scam is complete. The victim sees the money in their bank account after a few days, and has no idea that the money order or check can be cancelled days to weeks later when it is discovered to be bogus.

I don't know if the OP is victim or troll, but I do know that the scam is more successful than you would think. Victims are embarrassed by their naivete, and frequently do not report the crime.