While the way a certain member handled the matter was poor, I think it does bring up some issues with the marketplace. A 4 day system may reduce spam to a degree but is likely to lose the interest of many people, of which some might be valuable future contributors.

Perhaps an 'Apply for Marketplace Membership' button would be more sensible? Ask the user to explain why they want to be involved, and what their first post is going to be. It would take a moderator to approve any applications but that is without doubt the best way to decide whether or not an applicant could become a useful member..