I think you need the minimum post requirnment for a few reasons:
1. To reward regular contributors with first chance to see any bargins being offered for sale
2. To ensure sellers are trustworthy and not dumping crap and running.  (I know most people on this site would never do this including the lerkers, however, you need rules in place to assist in protecting regular members and contributors).

Like has been said, it does not take long to rack up 20 posts...