Looking from the users profile, I am assuming he is an account seller. An account seller can not be trusted. Seeing he have no history on the forum, you really needed to use a trusted escrow for the deal. It could cost a little fee but you would not need to lose that $400 which was a good amount of money for you.

However, you need to provide some evidence so that everyone can justify your case and leave a negative feedback for the user. Right now it's just some words you left for the accuser and asking everyone to believe you.

Sorry to hear your loss if that's indeed happened.