As cheap as I generally am, the ONLY system I'd NEVER buy used is a 360. With how many problems that thing had, you're honestly better off just buying a new one at this point, and hope that you get one with the new heatsink.
Plus, does Microsoft's RROD warranty cover used? Does GameStop not put their own 'warranty' sticker over the Microsoft one? Am I making any sense?
... *crosses fingers*.