Couple issues:
You haven't explained how the trade happens without 3rd party.  This is a non-trivial problem.
Buyer has USD.  Seller has BTC.  They agree on an exchange.  How EXACTLY do you intend to faciltate the transfer so that Seller's BTC go  to Buyer's wallet and Buyer's USD get transfered to seller's Dwolla account without one party just taking both and leaving.

You haven't explained how you will enforce bids without a 3rd party.  This is a non-trivial problem.
I offer for sale 1,000,000 BTC @ $1 ea.  How do you ensure that I have 1,000,000 BTC and that I can't remove the order once matched and executed. 

There likely are solutions but you seem to have glossed away the entire problem which isn't reassuring.