how counter-party risk is mitigated on such a exchange i.e. what are the chances that after I transfer my bitcoins to some anonymous guy I get any fiat in return?
Currently there is none. However, Dark Exchange has a concept of identities which link a user name to a public key/private key. All messages between clients are signed (which seems to be what's breaking with bouncy castle at the moment) and you can view the public key before accepting an offer from someone. This gives an incentive for people to guard their identity and build up a reputation.
If you're exchanging with someone you don't know and have never exchanged with before, you probably don't want to exchange large amounts. Once you become more confident in that person, you can start exchanging larger and larger amounts.
In the future, I hope to implement a web of trust like feature on Dark Exchange to help people determine who they can trust.
Score users would be nice. More trades and more volume with diferent users, more score, registration date to make more scores too (or not?). If a "bad guy" appears, just put negative scores. It not will be the best way, but make some reputation.