Limiting it by post counts is silly... you can rack up several hundred posts in a day and they could all be drivel so it is not really a decent perimeter.
I remember that a year or so ago there was talk about a time limit before posting a for sale item say like 3 months membership, but allowing "wtb" ads in that time that method if the software can auto-magically manage it would be ok.
so would an honor system of 1% per item donation system for everyone when they sell on the classifieds. the fee would be less than than ebay/paypal rape and while not generating a lot of cash (so far the total sales there is ~300k so the take would have been 3k)
But really this brings us back to the paying member issue... many are willing to contribute to help grow and maintain the site even if they are unwilling to subsidize things like blogs and emails.
perhaps adds form newer members could go up as "pending" and then needing a mod or admin to look at it and allow it through... more work for the unpaid staff :p but they all claim it is a labor of love anyhow :roflmao
that is my plug nickel on it.