Thanks for the reply but in this case, before starting to bid, the user had a negative balance... And he won the two auctions.

Moreover, when I try to bid more than what I have, the system doesn't allow it to me, it opens the purchase page to buy credits.
Therefore, if I can't, why would he be able to do it? We are not talking about cents here, we're talking about 20 Millions which, for a level6 user, is a huge amount.