Price, location, alternative way to contact the seller. Let 'em list, that's fine, but don't ask them into the house unless they're invited.
IMO - no minimum posts, that seems silly. You want to post something here for sale? Great. No prob. You take a cut.
I just sold a Bass on Reverb.com. They let you list for free, but if it sells on their site they get cut. It's small. You have a captive audience, I sold a Rickenbacker 4001 that I've had since I was a teen. I listed, their terms were spelled out. People are allowed to make an offer. If that offer is accepted the seller is obligated to sell it. The buyer still has the opportunity to back out. The seller has the right to reject the offer. No fees are paid to the site until the item is sold.
It's a nice system and everyone seems happy. You are listing to an audience that understands what you're selling, and the site gets a little help.
I would check that out. It ain't eBay or CL, and seems a lot more civilized. Or, as the Aussies and Brits say, civilised.
EDIT: It may be a nightmare for the programmer, but anyone with "real" membership - i.e. member-to-member, is exempt. Maybe create another section on the site? Like "Outside Sales"?