I don't understand the obsession with decentralization. It might be useful but there are many successful financial exchanges that are centralized.
Risk of failure of central node and need to eliminate escrow(trusted third party).
Decentralized trading is not possible with fiat currencies as they themselves are fully centralised.
Have a look at my proposal of decentralized trading between alternate crypto-coins.
https://bitcointalk.org/index.php?topic=112222