You would create your own altcoin called Derp's USD coin and peg it to USD. Then you are now the gateway to cash fiat out of the network. They pay you to get Derpcoins and you hold money until somebody cash's out. People trade your altcoins against bitcoin and it acts as a voucher but secure. You would obviously mine these yourself. Could also use Open Transactions to do this. Whoever wanted to trade Derpcoin with you then could.
That's a promising idea. I like it, but I am skeptical on how you could possibly bootstrap a coin pegged to USD. It would not be an altcoin based on a blockchain, just signed by an authority who guaratees the value. There are some obvious trust issues here.
But I'm all for it in principle, as it would empower the decentralised exchanges. This is the core problem to solve. Focus on this, not on how to build yet another useless peer-to-peer web framework. A thousand of those would bloom if there was a medium of fiat exchange that could work decentralised without the risk of chargebacks etc.
various fiat gateways around the world would release their own vouchers/coin pegged to fiat who had otr ratings and were trusted enough to use, then in the exchange users would just p2p trade between themselves through escrow nodes/bots run by anybody with trusted rating, or between users who trust each other enough to trade and are gpg identified like what happens everyday on #bitcoin-otc.
mtgoxusd/vouchx is the same thing basically. if you had one of these vouchers or exchanger coins you could cash directly from whoever issued it or trade it p2p with somebody else looking to buy in and get coins. ripple sort of works this way too, except it's closed source .
now you can write a bot and have it buy/sell automatically for you across the network using escrow bots. jack up the anonymity level on it (choosing your level of anonymity is actually a gnunet feature), configure stop loss, and it could trade safely since nobody could find it. it would only be identified by it's gpg authentication handle/ratings and resolved GADS name through dht, which other traders could look up, which hopefully had some sort of contact info if there was a problem. because bitcoin does multisig cold wallets you could run a separate security bot to watch and verify trades with a second signature to move the coins, either automatically or it sends a signal to user to manually initiate trade if they are satisfied.
on the users end they would just enter a price/market/limit and click sell and bot would do it's thing until it found a match based on pre defined trust levels and prompt you if you wanted to proceed or configure it to auto trade and let it go for awhile trying to find the best price.