Well, we don't seperate the payment of ads by usergroup; we set payment by category. To exclude some groups from the payment would be some simple code like: if ( user in group) $payments = 0; kind of thing. Not much work at all.
Ads can be posted for 3, 7, 10, 15, 30, 60 or 90 days.
http://www.photopostdev.com/classifieds/index.php is an example of an integrated vB3 forum with Classifieds. You can also view the user submitted sites in our
Sites In Action area. Classifieds uses the same headers/footers and styles as your vB3 site - so even your skins will be picked up and used by Classifieds.