I don't know how much players would appreciate/accept this, but how about 24h time until the items get transferred actually?
As soon as a deal is accepted, both items/money get removed, but it needs 24h until the items actually arrive at the other player. Within this time both players can cancel the deal, and once one of them decides to do so the deal is being reverted immediately. Perhaps you can make the termination cost money (a percentage of the item value?) for the player who cancelled the deal, transferring it to the other player, to compensate a bit for the lost time.
Players shouldn't be able to skip this time, or you would have this problem again, because in the moment they accept the deal they believe it is right, and they will confirm any question, if they REALLY want to make the deal or not. Just let them wait 24h, it shouldn't hurt them too much.