This would be really cool IMO, especially if groups and such were supported - This could even serve as a platform for conducting P2P trades with DRK. However this isn't something we should be thinking about just yet, development is currently focusing on improving DarkSend, then moving on to InstantX. Once these are done, we can start discussing things like this, services that we could use the network of MN's for.
But yeah this is one advantage Bitcoin really has over DarkCoin: P2P (read: off the exchanges) trading - LocalBitcoins basically. DRK really needs a platform like it. I'm actually considering making such a site for DRK myself when I finish uni, both to advance my web dev skills and learn how to apply them in the real world for crypto purposes. Not promising anything, but it's going to be the first project I start on. If it turns out good and proves functional, I'll definitely put it up.