Allowed = Yes, but it's strongly recommended that you don't do it since it's a high risk trade.
However, if you must do it, then it's best get a Game Sage or Game Master to assist with the trade.
Not only does this scare away most scammers, it also provides a layer of protection. A game sage or game master can hold the money to ensure the buyer has it, and also to help ensure the code isn't fake.
While this still isn't fool-proof. It does provide a extra layer of protection. Again, Aeria does not encourage this in any way.