To be honest what is the difference between selling links on your site and becoming a publisher for an ad campaign displaying banners? Isn't it the same. That being said do google not like ad networks either? Or is it because ad networks generally dont pass PR?
I don't know much,but read somewhere that,Ad network ads are the javascript hence not affecting the PR of site you are linking to,but by selling links you give direct links to the Advertiser which can help improve their PR,Google is strongly against this can penalise your site if google find out that.
What Google doesn't like is buying or selling links in an effort to manipulate the search engine results. They don't mind if you buy links for the traffic. So ad networks are o.k. with them. If a purchased link has nofollow google sees that type of a link as an advertisement and as a result is fine with it.
It makes no difference. But Google don't like the networks. See what happened with Text Link Ads for example...