Backing out of a deal that is in progress no matter what circumstances are at hand is a bad trade. He has the right to voice his opinion in other trading ads you post warning others about what you did.
Believe me, if you were in his situation you would be pretty mad and would like to warn others that he hasn't always been a great trader. Through ebay if you back out on a deal, a complaint is then filed. After the complaint goes through, the account is banned. We are not that harsh here unless it is a really bad trade