on another forum that i recently stopped working for, we had a rule for 50 posts to access the classifieds section. we had the software set to automatically promote the member to classifieds access once they reach that goal. 2 things happened, 1 we started getting laot of PM spam and members trying to bypass the rules by sending PM's to others who had access asking if this or that was in the classifieds section. it became very frustrating for those who were receiving random PM's with such requests. then, our other problem was that people would sign up, make 50 posts that say "welcome to the site" and then whore up the for sale section. this caused our moderators to have to work double time. now not only are they trying to keep all the ads in that section, they are also trying to squash those who are post whoring just to access the classifieds.
i have seen members with 1000's of posts screw other members. it has nothing to do with postcount. its a risk you take when buying/selling with unsecured websites like forums and craigslist. since starting up our new forum .... we have went with a no fee/no minimum postcount policy and things have been absolutely great for all staff and regular members.
as long as you follow a simple policy on selling/buying you will be ok.
1. NEVER send an item to someone who says they will pay you after its received.
2. ALWAYS get a shipping confirmation/tracking number when you send something.
3. ALWAYS use a online funding tool such as paypal when buying something. that way if the item is not received, you can at least attempt to get your money back that way.
i personally try to go the extra step to ensure someone buying from me is confident. i even offer to list whatever they are buying on ebay so they can go there and buy through there. that way, all my info is on file through ebay and paypal will always cover non-payment/non-shipping disputes with ease as long as you can provide proof of shipment and proof of payment.