I'm not a lawyer either, but I do know that a contract requires the agreement of both parties, and placing an ad does not constitute agreeing to anything. And while it may be annoying to a buyer to make a full-price offer and then see the price hiked, that is well within a seller's rights, and I don't see anything unethical about it. Of course, if you're annoyed you don't have to do business with the guy.
Also, CW, if you will permit a mild criticism, why are you posting? It doesn't look like you followed up with the seller to find out what he was up to. That should have been your first step, before taking this public. (If you did and got no response, I apologize, but you didn't indicate that in your post.)