Who said you can't sell text links for advertising purposes? Of course you can. Google isn't stopping you. Matt Cutts, head of Google's antispam team, has written:
'What if a site wants to buy links purely for visitor click traffic, to build buzz, or to support another site? In that situation, I would use the rel=”nofollow” attribute. The nofollow tag allows a site to add a link that abstains from being an editorial vote.'
So go ahead and sell advertising links. But if you want free traffic from Google, have the common sense to respect Google's guidelines on how to implement those links.