I love it except for one minor detail that may create problems.
Say for example someone does in fact clicks the button and then decides they don't want to purchase the item. What will take place? I mean like eBay

for example once you use Buy It Now, technically the same deal, if someone doesn't pay, they force being left negative feedback and the seller has to re-list or re-post the item.
So with this, are we making the buy/sell relationship that strict whereas the person buying will in turn have no choice after they hit the button and confirm, and they must purchase or have no choice but recieve negative feedback.
My main point is how strict in fact the button will be. I love the idea of after clicking the button it closes the thread and PMs both users but I just don't want people to be forced into deals as a result of clicking the button.
I guess actually maybe it won't matter and this whole post was meaningless because as long as you can be clear with how it will work to your forum members, it shouldn't be that much of a problem.