(Full Disclosure I am on the team at BSA)

At BuySellAds.com we make it really easy for independent publishers and content producers to do exactly what you are trying to achieve.

Check out how easy it is to embed scheduled sponsorships to your site. http://blog.buysellads.com/2013/new-bsa-pro-feature-scheduled-sponsorships/

We also allow publishers to install a sales widget featured here that combines all the pertinent demo data that Charlie mentioned. ex. http://www.cultofmac.com/advertising-with-cultofmac-com

This allows advertisers to schedule and sponsor directly from you.