I think it's not a good idea to leave the question unregulated... If a gamer finds himself in a situation that force him to get his player back from transfer due to injuries for example it could be easier to add an option to get the player back paying a small fee... in this way bidding on owned player could be declared illegal.
By biddin on a player of mine I can fake the final price and also by doing this in the end of the bidding i can extend the bidding time by 10 or more minutes!!
If you win the auction on your own player, you are already paying a small fee because you don't get to keep the entire auction price. Additionally, you'll get an even smaller percentage of the auction price when you decide to relist the player. And, finally, I think that most people would stay away from an auction if they saw that someone had bid on their own player. So, you might be able to extend the deadline (though I fail to see what that would accomplish), but you'll get stuck with the guy and wind up losing money.