Personally, I think that you first try to resolve the issue with the seller/buyer. Sounds like you did that.
If still unresolved, then - without emotion or emotionally charged language - present your side of the story (the facts, not conjecture, and not your thoughts on what should happen. Just the facts.). The other person is free to do the same.
You'll get plenty of responses probably, this is, after all, BPL, and there are always lots of opinions floating around. Some will probably be 'on your side,' some will probably not. Nature of the beast.
But it does let everyone know of a possible issue of dealing with someone via the gear swap on this site.
(WARNING: This statement has not been evaluated by BPL. This post is not intended to diagnose, treat, cure, or prevent any rancorous diatribes from other posters)