Building an online classified ads business is proving to be a very successful concept. While people generally expect to be able to post text-based ads for free, you can generate income by allowing people to post enhanced advertisements (e.g., ads with pictures) or classified ads with hypertext links to a Web site. Additional revenue would come from direct advertising sales (e.g., banner ads). An option you may wish to consider is setting up an e-mail system for your users, where they can register for a free e-mail account. You could then send them advertisements based on their opt-in preferences.
Coupons, auctions, real estate information, and so on, are all related topics that would be appropriate for your site.
If you need more advise to setup this business, please contact me at firstname.lastname@example.org