There is a similar site [1] without external incentives (actually Proof Market is based on this). This site has active audience posting problems and proofs. Possibly Proof Market ends up in another example of overjustification.
This gives very broad perspective. I want to cite this comment when I talk about the site. I have never thought about disrupting Wall St, but I do share your pipe dream.
I can mix both approaches. The buyer and other people can stash up bounty, which the first prover gets. I have not implemented this lest "bitcoin stolen from Coq proof exchange".
My priority is on keeping the formal definition in sync with the Yellow Paper and the implementations.