A friend of mine suggests an interesting idea: player-created quests. Players would specify the object of the quest: the object to be returned, for instance. ("Bring me the broomstick of the wicked witch.") And they specify the reward, whether it be money or objects. Players can sign up for the quest (or maybe they don't have to?). There could be an in-game escrow system in which the creator of the quest deposits the reward. A player would brings back the object of the quest and deposits it into the escrow system. The escrow system gives the object of the quest to the creator of the quest and gives the reward to the player would brought back the object. (Hence there could be no cheating.)
The originator of the quest could use the quest as a way of getting a rare or valuable object by paying for it instead of finding it himself.
The quest listings (and escrow system) would be analogous to the current auction system. They might even be extensions of it.
This might be expanded (later) to quests involving more than bringing back a specific object. A quest might, for instance, include a number of places to visit, skills to learn, or things to make.