Cool project. We do something very similar with an internally made lander for our domain portfolio, and it works well. Lots of serious buyers contact via e-mail listed on whois too.

One thing very useful is to have analytics, perhaps you could add a feature for users to input a Google Analytics tag (UA-000000).

Side note: for anyone who actually wants to sell/buy a domain in a private transaction, I highly recommend Escrow.com as an escrow service.