There are already very successful domain trading platforms around (http://www.sedo.com). I can't really figure out what namebox's unique value proposition is.
I used to have my email address listed on some domains I had for sale, and after rejecting a ridiculously low offer ($40 for a 3 letter .com) had some 14 year old kid and all his buddies emailing me and calling me at all hours to try and harass me into accepting it.