It depends upon the theme you use. 1. Music 2. Arts/ pictures. 3. Movies 4. Softwares/scripts Some have sold papers, presentations, e.t.c. All this can be linked to number of posts/threads and giving discounts over items. That is the best part of using forum with downloads.
You guys might want to check out this article.. http://www.vbshack.net/forum/three-out-box-t52/index.html I wrote it a few days back, but haven't had the chance to post it up due to the recent public holidays. Hope it helps.