I moderated on 2 forums and the buy and sell forums were always the most involved part of it. Scammers always trying to skirt the rules and some users only wanting to be a member of the forum to use it like Craigslist.
One of the developers who worked for the owner explained it to me like this:
Forums cost money to run. That is paid for by membership fees as well as ad revenue. If members post good content the advertisers pay.
So a non-paying member pays their way by posting quality content. Someone who is only here to post the minimum in order to use the S&S forum is not helping the forum at all, just using it for their personal gain.