My opinion is that the absolute BEST thing you could do in this instance is to post the seller's name, eBay ID, and any other details RIGHT HERE on The Chip Board for all to see. You'd be doing a service to the collecting community in general (in a much more effective way that just posting negative feedback).

I've never lhad cause to leave negative feedback, but what I've always thought of doing (and I really don't know if this would be allowable), would be to just put something like "If you're thinking of doing business with xxxxxx, e-mail me at xxxxx@xxxxx.com" This way, there are no details the seller could refute.