Google doesn't like link selling and punishes sites who do.
What's the difference between selling advertising space on your website (banners and text ads) and selling links?
3:26 pm on May 15, 2008 (gmt 0)
Advertising is selling actual traffic (or impressions that could turn into traffic) while link selling is selling "link juice" trying to manipulate Google's organic listings.
3:41 pm on May 15, 2008 (gmt 0)
And how does Google tell the difference?
2:11 pm on May 16, 2008 (gmt 0)
I'm not entirely sure... certainly all ads that use no-follow will not give any link weight. People can report paid links to Google. Also Google could be identifying ads by many factors including: manual reviews, position on page, text near links/images (ex. - "sponsored results"), etc.