From Matt Cutts Blog –
let me talk about why we consider it outside our guidelines to get PageRank via buying links. Google (and pretty much every other major search engine) uses hyperlinks to help determine reputation. Links are usually editorial votes given by choice, and link-based analysis has greatly improved the quality of web search. Selling links muddies the quality of link-based reputation and makes it harder for many search engines (not just Google) to return relevant results. When the Berkeley college newspaper has six online gambling links (three casinos, two for poker, and one bingo) on its front page, it’s harder for search engines to know which links can be trusted.