You need to leave a "seller" feedback for them, the feedback system is crucial, doesn't look like you have left feedback for him yet. The risk of course is when leaving negative feedback you might earn retaliatory negative feedback, but I don't know what else you can do. This forum in my experience has the best members and is my favorite, largely a credit to Len I think. Unfortunately this can happen even on this forum and I don't think I would expect Len or any admin necessarily to get involved, they can't really do much more than you, other than ban said member from the forum, but that doesn't get your 100 bucks back. The best way to minimize these issues and to keep it honest is to leave honest feedback. Might be helpful to screen shot and save the original posting, and his message to you FWIW, as he can delete all of that and pretend it never happened.
I know it stinks that you'll end up losing 100 bucks on this deal, be thankful it wasn't 700, but also remember that while you can't do anything to get that back, 100 bucks doesn't change your life, meanwhile if the other party did as you say, they sold their integrity for 100 bucks, most of us would say integrity is worth far more than that.