How would the OpenBazaar or NXT decentralized marketplace / eBay / Craigslist work? Since NXT and OpenBazaar are building a decentralized marketplace, how would this work since there is no middle party to mediate if an issue arises?

What are some proposed solutions to a decentralized marketplace so far?
One solution is to use Nash equilibrium in order to force people to cooperate.

The concept is quite simple. Both the seller and the buyer make a deposit.

If the entire buy/sell process goes fine they can recover their deposit, if it don't they both lose their deposit.

So it isn't profitable to go away with the money without sending the good nor to get the good without releasing the payment to the seller.

This solution has already been discussed. If you google bitcoin+nash+equilibrium you will find more informations.