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As with the marketplace itself, I think the best way to prevent non-marketplace threads from popping up would be to enable everyone to click the link to the marketplace. Of course in this case non-eligible members would be directed to a page stating why they are not eligible, including the FAQ link to the reason, a link to the marketplace rules, and link to the FAQ for the forums as a whole. That way people know a marketplace exists, promotes the FAQ, so threads on why there is no marketplace do not show up. At the same time eligible members would be able to access said forum directly. However for the suggestion here, I don't think vB lets you link the same forum to two links based on eligbility, so possibly everyone with less than 250 posts and 180 days would only view one version of the forum (that says they are not eligible), and every one else (who is eligible) would see the real forum.