I don't see a reason for it to be against the rules.
However I must warn everyone that there is always the possibility for someone to scam you; so be on guard. For instance you might give them the banner and then they won't pay you in game, or vice versa.
My opinion, be careful and make sure you trust the person your selling something to.
(if another administrator see's this and thinks that we shouldn't allow this, feel free to lock this!)