I don't think bribing, ie convincing, another player to perform an in-game, ie role-playing, action, should be a bannable offense; ok not sure on this, but whatever. It at least isn't bannable under current MMORPG terms of service. All it says is that the company owns the servers, the game, the service, any property in-game, the terms of service, etc. Well no matter how much money you shove into another player's pocket for gold, the company can always pull the plug on the game and its in-game assets. There's not a damn thing you can do to argue that it's your gold and that they can't get rid of it.
Issue solved, right? All MMORPG companies need to do to eliminate bribe money is to add in their terms of service that a player can't give real money to another player to convince them to perform some in-game action. Now they need to enforce it ;). They could always just allow limited gifting, or get rid of it altogether.