This "decentralized trading" idea can only work with digital assets (e.g. blinded Bitcoins...), not fiat money. There is no way to "digitalize" a dollar bill without introducing a third parts (called bank, escrower, gateway, exchange...).
Here is a solution using collateral to build a system for trust-less fiat-btc trading:
https://bitcointalk.org/index.php?topic=462236