Jane_Doe - 7:07 am on May 29, 2010 (gmt 0)
I don't know much about moderation to give you specifics but check out some of the competition and see what they are doing. Maybe allow dofollow signature links. A lot of people like that. And have more commercial forums specifically meant for buyers and sellers of sites and services.
I like the idea someone else posted of having a site for sale forum. If you could find some way to keep it to higher level than flippa I think that would be a huge draw.