Have you addressed it with Admin? Certainly they'd be able to confirm your transaction of a GC. I would think they'd then be able to see that he used the credits to join the other world and not yours.
The reason I say so, is that you haven't fully gone public with it yet. If I were Admin I'd rather you took it up with them first, rather than just paint the guy's name on the wall here. He stole your money. You have the right to report it to them, and if they can't settle it for you, then we have the right to hang the guy in public.