You would not need to build an ad network. There are a few hungry ad networks out there already that offer text ads and have systems similar to this already in

You would not need to build an ad network. There are a few hungry ad networks out there already that offer text ads and have systems similar to this already in place. The only challenge really is in categorizing the sites where the widget appears (to target the ads so that the CPM ratio doesn't make it worthless). Past that, it's really just seeing who has a good base of customers buying text ads and letting them handle all of the dirty work.