Rikishi sold it for The Rock.
No but seriously, that is a good idea. The only downside is, how do you justify someone selling off their MITB contract? The whole idea of the WWE Championship is it is in it of itself the ultimate prize. Everyone should want it more than money, more than anything. Plus, until it's cashed in, and the reason for selling it can be made clear, that cloud of "Why" is going to hang over the head of the original contract winner, and might well kill any heat they have...