"You asked for $500 for 2 or 3 names. I offered $550 for 3 names. Deal..."
I'd say no deal. I think you were trying to take advantage of the fact the seller's first language isn't English by trying to nail him with the $550 for 3 PM.
Obviously, he meant 2 or 3 at $500 subject to negotiation and agreement not that you would pick the 3 you wanted and fire off a PM to claim them for a token $50 more than he was looking to raise.
In the seller's mind, he'd sell you 3 provided they weren't the 3 strongest domains otherwise it would be 2. He didn't mean 2 or 3 literally otherwise he wouldn't have had many takers for 2.
Even if it was a completely innocent misunderstanding on your part, you should have done the obvious thing and confirmed the deal you thought you'd got in the thread and by PM.
I think this is getting blown out of proportion. The best solution is for all negative feedback to be cancelled. The seller has to tighten up on his offer phrasing and the buyer should confirm his take on offers accepted by PM and in the main sale thread in future to avoid misunderstanding. That way the eventual buyer could have waited until the misunderstanding was resolved amicably before his improved offer complicated things.