I like to hear the negative things as only positive comments seem insincere or fake to me. No one is completely happy 100% of the time.
I ran something like this on a local forum and thought that making people stick to
only first hand experiences and with
only the facts, that more productive results came from it. Also, the intention of the thread/section was for awareness/resolution and not to discourage or harm the others involved. Posts from people that say their "friend was ripped off", or even first hand complaints that don't specify details were either disputed for more details or closed/deleted etc. Sometimes asking for specifics showed that they weren't totally innocent either. Make it known that the vender has equal opportunity to respond as well. They also needed to be legitimate without being freinds with, or working for the competition.
Feed back like what the have on Newegg is awesome and the legit comments on there have been a huge factor in purchases I've made there. -great feature if you can pull it off.