way too many "resellers" creating new topics for every different niche of wtb & wts. you can have 1 user who create like 5 new posts for travel / health / fashion / etc etc maybe can limit to each user one post every 12 hrs on the link sales forum ? premium members probably a couple of posts more of course. just my 2 cents.
Probably will be better to simply require entry of the site the links are being sold on, and then you could programmatically disallow duplicates.