People will pay money to advertise on other websites. How much money they will pay will be a factor of many things:

1. How popular is the site? (traffic stats)
2. WHat kind of advertisement is it?LinK? Banner?
3. If banner, what size banner?
4. If banner, how many impressions?
5. Is the site related to their own site (It would be useless to advertise webhosting on an adult site for example)

If you want to sell advertisements on your site, first of all it must be worth advertising on. Advertisers will want to see a return on investment. What do they get out of advertising there? Is there any chance someone will click the ad and visit their site? More importantly, is there a chance they will by something from the site? If your site gets users that are "targeted" for the advertiser then it'd be worth it for them.