Marlin, I am not certain why post count has anything to do with the ability to follow through with a deal. It could certainly hurt the be detrimental to the forum in that if someone needed to move an item quickly, say to cover their mortgage or whatever else in these uncertain times, a minimum post count would only force them to seek some place else and deprive the people here from the ability to acquire the item.
Is there a moderator who only watches the Trading Post? Perhaps a contract, verified by a moderator and a watch list of pending deals in a sticky would better serve to quell some of the broken deals, etc. a little public accountability does help to ensure a higher standard. I honestly can't see how a certain post count would affect a change.