When I was thinking of how to set up a distributed marketplace, I came up with Proof of Burn myself.
The idea would be that you want sellers to pay something to participate in the market, but it's not fair for them to pay it to a central authority. So the solution is to burn some coins.
I should really put together the rest of the ideas into a whitepaper or something... There's a way to do a distributed marketplace in a fair and reliable way. There are a few people trying to do that currently, but most of them seem hung up on some weird design decisions.
Mainly you want to be able to see how many orders a seller has "in flight" at any given time. If 30 orders are placed, and the seller hasn't processed any of them, it's a good sign they're going to welch with the coins. The fact that certain currencies are publicly viewable (BTC for example) allows buyers to have those insights into sellers' activities in a distributed fashion.