not sure if there is anyway to enforce it. In the past I have pm'd people when I have noticed the cancelled deal, but only if something seemed awry.
Realistically once a deal is agreed to by both parties it should be completed - if not then appropriate feedback should be left. That's what the feedback system is for.
I have in the when I've felt we were on the same path, put up trade agreements early to get the ball rolling. Such as "so I'm going to sell you these cards for x amount, if your happy with that here's the trade agreement and my Paypal details are this"
I find it can just speed up the process a bit, but then sometimes there's a misunderstanding or whatever and I have to cancel a deal. It's no biggie.