The concept is nice.
But I think the option would be abused.
People already troll frequently in the account section. With a bidding system there would be so much controversy and negative feedback etc... for example if someone bidded on the account and decided they didn't want to buy it, but they won the bid.
Also I'm not entirely sure as to how this would be implemented on the forums, unless there's some sort of bidding plugin.
On the contrary, it would make the process of making an offer for an account easier, and would also mean you don't have the seller falsely telling you people have bid on it so that you have to up your price offer.
I'll have to remain neutral on this one.