Why couldn't there be a DRK/BTC decentralized exchange? Use the masternodes to host the portal of buys and sells (when an offer is agreed on, the ad is removed from the portal), when someone wants to buy/sell, each respective party puts up their side of the coins to the masternode network, once both send their end of the deal, the masternodes exchange to the other party. In the event one party backs out, the masternode sends it back to the original party. The portal would show up in each client wallet under an exchange tab. Essentially, the MN network acts as a built in escrow.