Possibly the best way to moderate this is through a specific PermaThread, rather than additional postings. Guest postings would not be allowed, and the opening post of the thread would warn people they would be deleted (oo-er!) if they were not members.

People should change the subject of their post to reflect what they have for sale, which would facilitate searches and would also show up via the 'Messages since last visit' feature only available to members.

If the person selling then does so via their own email, it shouldn't affect Mudcat traffic appreciably. Posters would include their email address with _at_ substituted for @ to deter spam harvesters, which would go a long way to ensure only interested parties used the facility.

In other words, a very minimal change to the Mudcat infrastructure...