I like the idea of some sort of rule for posting 'for sale' ads... but post count won't cut it. As has been said already, many folks will just post 'ditto' or 'yeah, you're right!' over and over to boost count.
I am personally familiar with that practice, as one of my dearest friends with whom I play two online D&D games will put up a post to ANYTHING just to get more roleplaying experience points. Luckily, the DM (my BF) has noticed and will no longer be rewarding him points just for posting.

Anyway, back to the topic at hand...
I believe that research is the greatest tool anyone can have for any situation. It is up to all of us as individuals to notice if the person putting up the ad is
A) Someone who has been seen around the site before or not. i.e is that their first post.
B) One who has put up ONLY for sale posts and nothing else.
C) One who doesn't answer PMs or answers with questionable things.
D) One who isn't willing to talk with you offline (at least by phone) and give you details of their animals.

Other than that, if they give a name check it out on the BOI and ask around. That's what the Inquiries section is for after all.

Scammers get to people who are not alert, who don't ask questions, and who don't know their way around that particular business area. They are clever and crafty folk who stop at nothing to achieve the scam.

I agree with the idea to have an Adoption section, as many people just want to find good homes for their animals for whatever reason.
As far as the for sale section...maybe a membership time minimum would work, but the best weapon is alertness and research.

Sorry for the long rant, there.