This would cause so many problems, I can't begin to list them all...
1. people placing huge bids on players to eliminate interest from the player, withdrawing the bid later and putting a much smaller bid on it.
2. people spending money from one of their "for sale" players, only to find out they didn't have that money.
3. people bidding on rival's players to eliminate interest and then just withdrawing bids to screw them over.
etc.
Steve