Fine. Shut it down. It is worse than eBay here if there is no way to look up historical negative feedback on a seller.
Let readers sort it out if a seller is being unjustly harassed by abusive negative comments -- I have faith they can.
Based on my experience, and I have been a moderator of another forum for a long time and have help investigate a number of issues on various forums and newsgroups, your faith is misplaced. I cannot think of a single case in my experience where the correct outcome actually occurred. In all cases, the outcome was distorted. My investigations into those cases were only possible because I knew the parties outside the forum and was in a position to ascertain the truth. That did not happen inside the forum, and when I reported what I found, many just didn't believe me. In every case, the outcome favored the person who could make themselves look more sympathetic. I learned the hard way that there is just no way to make it work. There are already mechanisms in place to arbitrate disputes, if they are worth arguing about.
Rick "good enough with words to know when not to trust them" Denney