It is listed on the Board Rules page, for which there is a link at the bottom of every page. A new user needs to have 25 "approved posts" to gain access to the For Sale forum.

The reason for the addition of "approved" is that the rule is designed to restrict access to those who have engaged in useful activity and productive conversation in their postings, not just banging out a series of 25 of "I agree!" or "Good idea!" posts that may as well have been posted by a spambot.