Does anyone know more about PAUV?
Seems very interesting but its still in dev.

http://pauv.org/
http://coinrevolution.com/pauv-brillante-proyecto-exchange-descentralizado#.UslgePbOYpM

They use the Nash Equilibrium idea to use a collateral:
"Informally, a set of strategies is a Nash equilibrium if no user can do better by unilaterally changing his or her strategy. To see what this means, imagine that each P2P user is told the strategies of the others. Suppose then that each P2P user asks himself or herself: "Knowing the strategies of the other P2P users, and treating the strategies of the other P2P user as set in stone, can I benefit by changing my strategy?""

And payment processor APIs:
"Also a payment processor that works with local currencies and that has a public API is required to make automatic this operation. Actually there are many payment processors that have a public API, as OKPay (www.okpay.com) PerfectMoney (https://perfectmoney.is) WebMoney (http://www.webmoney.com.mx) , etc., These services allow transfer money in your service account to other users using an API, and they don't allow any kind of chargeback."

And a clever combination of MultiSig with ANYONECANPAY for the collateral processing:
in : {

}
out : {

2BTC output <H1>
2BTC output <L1>
1BTC to <H1>
3BTC to <L1>
}
<H2><L2> CHECKMULTISIGVERIFY
[MESSAGE: HOMER PAYMENT PROCESSOR ID, MONEY AMOUNT AGREED]

The paper is really worth reading!