Indeed, Deloth, it appears that you have been robbed. It's an ufortunate part of life, but it's not against the rules of this game. (Personally, I was surprised to discover in an IRC conversation a few days ago that there are games where theft is against the rules.)
Unless you've got the money to put a bounty on his head, your only recourse will probably be to start a thread in the Trader's Forum so others know he's a thief. Beyond that, consider it a costly lesson on the risks of trading in the Combine (hope it wasn't too costly, though -- you seem like a nice enough fellow).