I would:
- Require 50 posts before allowing access to the classifieds
- This allows a user to show their level of involvement through direct participation rather than monetary compensation
- Require 50 posts before being able to use Private Messages
- This cuts down on users circumventing the rules
- Waive both requirements for Site Supporters
- They've already shown a level of involvement with financial support, which is equally important to the running of a website