Not so much an answer to your problem. Human nature being what it is, most people are going to offer their place to a friend first off. They will post and offer to first reply or able to pay there and then. I have come across situations on the seat exchange i haven"t been too fond of, but ultimately, it is their seat to sell to whomever they wish, whether we agree or not.
Another reason that they post/sell, may be that they want it sorting there and then, as they do not have the time to trawl through posts and await replies.
Suppose we have a first come first served list, and three seats become available. How long do we wait for replies from first three before going onto 4/5 or6 etc. Now suppose 2 replies but not 1/3, we allocate seat to 2? then find seats 1/3 are no longer available for whatever reason. Now 1 may kick off as he was first request. I can think of a lot of perms where this could falter.
Sell back to Apat? This would not work, as they want the seats sold(not bought) also like above, the time and confusion with re allocating seats etc.
So i think were stuck with what we have at the moment, and people requiring seats simply need to monitor the forum, and not put the owness onto the seller. After all it is a sellers market at the mo