You don't have to sell or buy links per se. Further, you can sell advertising, to a targeted audience, not that pharmacy/enlarge/gaming totally unrelated to your site junk. That one of the underlying reasons for the advertiser to get an ad spot may be the PR value, it's OK, and as long as the ad suits the website, people will not be bothered by it. You can see from Matt's post the type of links they consider as spammy. The further away you go from that, the safer you are.