[gwern]

Depends on who you want to put the burden onto, the buyers or the sellers. If a buyer has to pay up front, especially with a marginal service like this, they're exposed to opportunity cost (maybe they were going to do something else with that money, although interest rates don't really exist for Bitcoin yet) and counterparty risk (why do you trust this website to hold onto your bitcoins and not be hacked?). If this market never takes off - as is one's default expectation - it'd not be pleasant for buyers to have paid up front.

[pirapira]

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".

[tsewlliw]

Way to avoid arbitrary risks and complications!

[pirapira]

I added risks and complications. A feature called "bounty" is now available. Anyone can add bounty for a problem. The sum goes to the next solver.