Some people seems not to understand a very simple fact :
As soon as a buyer and a seller get an agreement (whatever
on a buyout price or trade or whatever), the whole auction
stuff is pointless : Even if you were not allowed to
stop the auction, there is nothing to prevent the buyer
to bid whatever is needed to "win" the auction : he won't
have to pay that to the seller anyway.

In fact, it sometime pretty fun to continue the auction
when you know that you can "bid" whatever amount you
fancy as you already agreed on a fixed deal. :)