martinibuster - 4:54 pm on Sep 17, 2012 (gmt 0)
If you want to remain in Google's search index then you have to follow their quality guidelines. You are selling paid links, and when the links are dofollow it is seen selling PageRank. That's a violation of their guidelines. You can still sell ads, either image ads or links, but you have to add the nofollow attribute.
There are alternate methods for selling advertising, such as using OpenX or blogads.com.