I got this idea today I thought I would share.
Limitations to be put on the advertising forum: You must have 10 posts to post there (10 is better than 20 becuase 10 will prevent spamming, but will prevent first time users from posting). I also think that advertisements should be deleted  every 30 days, and users should only be allowed to post an advertisement every 45 days. I think we will probably have to create a special forum in the mod section where after we delete an advertisement we post "[Username] Ad Deleted [Date]"
But of course there will be some who will want their ad to stay permanent. Well lets offer them that too! For something around $5 a month, users can post an ad and have it "sticked" so its around forever.
We will probably have to work alot on these features, since we will need ways to communicate on who has paid, etc, but that can probably be done via the forums.

Anway what do you guys think?