Diablo II has "the cube" which allows you to combine certain items to produce a single item. While most of the recipes include low and middle level items, there are a few that require the super rare items like an SoJ.

Diablo II has also had world events where only a very limited few can get in on the action at a time. Specifically the uber Diablo event which requires players to sell SoJs to vendors (money sink) to trigger the event.

The real problem facing a game that uses items as currency between players is that if the game becomes to popular, you will have to fight duplication.

One OOB idea I've had is to create a much richer merchant experience - allowing for things like loyal customer discounts, research & development, et cetera. This allows such "money sinks" to become useful pieces of the game. For example the more money you spend at a merchant, the better stock they will keep. This can, of course, be expanded very easily which makes me think it to be a decent concept. (Just wish I had a game to implement it on)