Tell Blizzard you were trading for gold for gold on another server, this isn't against the TOS/EULA afaik, or atleast it wasn't in the early days when I did it.
Yeah that is why I tried to get him to chat outside the game, I'm pretty sure they will just check the chat logs to see I tried to buy a char transfer.
Super Unicum
Unless you see the guy on after he did it, you can't really do anything to screw him over. Honestly, next time you should only complete the trade if a form of outside chat is given and you get proof.