I hate to pipe in on this thread, but I do have an opinion on this, as I've thought and thought about it.
I do not believe a buy n sell thread is in the best interest of this site.
I think if you have something to sell, put an ad in your signature to help you promote selling it. The other side of the coin is that if you dedicate a forum to buy n sell, this is like condoning it, and when someone does a deal and everyone is NOT satisfied, there will be bad feelings, and someone might try to hold the site responsible, or atleast hold ill feelings towards the site. This is not only to protect the buyer, which is what we usually think of being ripped off, but what about some hardup, seller who actually IS ripping people off?
This could get out of hand very quickly, I like the ad in the signature approach.
Peace