These already exist. There are darknet firms that independently verify that an exploit is real (staking their reputation that they won't take it and run once shown), and then open it to the market.

The main trouble is that for it to fully work, you need to have the big corps bidding against black hats in this market. I can't see that happening. It'd have big corps dirtying their hands too openly.

The other trouble is that a lot of the damages (or value) of software bugs aren't readily fungible to dollars. For example, to actually profit from the Ashley Madison hack you'd need to blackmail a whole lot of people, which is incredibly time consuming. So software firms would be able to underpay for most exploits: The amount they'd be have to pay is at most what a black hat can profit from the bug, which is necessarily less than the real damages because the black hat has to cost for fungibility.