The straight up scammers get called out pretty quickly and their threads get crapped here and the other 3 or 4 forums where they have threads.
Minimum post counts are already indirectly enforced by common sense. The first thing someone is gonna (should) do before buying is look at the person's previous posts. If they are non-existent or come off like they don't know what they are talking about, that's an issue they need to work on, and they'll get called out for it. How they handle the pressure determines whether or not they still have a shot.
Someone coming in with 1 or 2 posts, offering a service and offering review copies should also use common sense when selecting whom to give them to. If they give out 3 reviews to people with 1 or 2 posts who are obviously all the same person, it's gonna show and they'll either get ignored or get ass-raped.
Anyone who takes more than 5 minutes to read some of the good sales threads and some of the bad ones will realize there's a number of members who you want to review your services (different people for different things) because they know their shit, and 1 good review brings 20 sales.
There's also the issue of trying to market to a bunch of marketers. It is very clear when someone signs up and starts a sale thread without reading the relatively short list of rules and pet peeves.
Also, be smart about this shit. If someone claims they are a SE0 and they guarantee to get you a #1 in google search for 'fleshlights', or if someone sends you 15,000 'real' uniques for $5, deal with them and you're likely gonna get shat on.