Minimum post counts will most likely just lead to spam posting to the amount required. I've seen it in computer related forums and it results in more work to check and make sure a member's posts aren't all spam, and in the end it really seems to do more to discourage legitimate sellers more so than scammers.

Srtforums had a big problem with this and it seems their solution was to require certain information be disclosed in the FS ad. Scammers are like roaches, they prefer to hide in the dark and tend to scurry away when the spot light is on them; however I can't see any reason why a legitimate seller would have a problem disclosing some basic personal info.

Here's part of what's required to post a FS ad at srtforums(hope they don't mind the copy/paste):