First, I would report him to the moderators of the forum. If he still refuses to sell it to you at the BIN price, give him negative feedback.
You are right. There is something wrong with it and most forums do not enforce the rules. The best thing to do is to report it to a moderator to see what they do. If nothing leave him negative feedback and move on. There is nothing you can do. It is not worth your time to pursue unless we are talking about a $xx,xxx or higher sale and you wish to get a lawyer involved. Anything less a lawyer is going to tell you it is not worth the time, effort, or costs involved.
obligated - Adjective - being morally or legally bound to do something:
Best of luck