On his Searchblog John Battelle writes about link selling. Quoting a post on O'Reilly Radar John questions whether it's appropriate to sell text links on a popular website just to help the advertiser taking advantage of the site Page Rank popularity. Should we consider this practice as search engine spamming? The discussion is open, and opinions differ.
If you have quite a few time and you're interested in search engine advertising, you should definitely follow the debate.