The reson on why it is not allowed is not just there is no way for the staff to check on it, but basically it breach the term of service of those said games, the details you could read it here: http://forum.toribash.com/showthread.php?t=91739
Also, the scam handling would be a hard case i would imagine. Not in the case when you give the tc but not didn't receive the said items. Toribash staff has no power to compensate when you lose the said item when you didn't receive the tc.
D) Promoting the trade of commodities unrelated to Toribash (aside from real currency) will result in infractions, and posts/threads being deleted (when applicable). Furthermore, you may not sell or trade your Toribash account under any circumstances.
I don't speak on behalf of Nabi, so take this how you will.
For a lot of cases, it falls under a failure to comply with a ToS of 'x' game. They don't want you selling their currency/accounts/whatever it may be, to a different game. But the most reasonable answer is, we don't want to deal with it. Even if it was a case of "if we wanted to", how could we?
We don't want, rather, shouldn't have to babysit people that can not trade fairly and responsibly. People are shitheads and can't have the decency to do things rightfully and fair; getting scammed is essentially going to happen.