How can you profit from a Web bulletin board or web classifieds?

I am interested in setting up a web page type bulletin board or classifieds on a topic not yet known yet...still brainstorming with ideas....In the long run, how can you profit from this type of site? I know craig'slist profited from partnership with ebay, I thought of donations to help keep the site running or advertising from companies of the given topic. I could use help in coming up with other ideas!!!! Thank you alot.|||This may be a weak business idea, these already exist on websites like facebook, and are connected with communities for better access and dissemination of ads.