Google states here :

“However, some SEOs and webmasters engage in the practice of buying and selling links that pass PageRank, disregarding the quality of the links, the sources, and the long-term impact it will have on their sites. Buying or selling links that pass PageRank is in violation of Google’s Webmaster Guidelines and can negatively impact a site’s ranking in search results.

Not all paid links violate our guidelines. Buying and selling links is a normal part of the economy of the web when done for advertising purposes, and not for manipulation of search results. Links purchased for advertising should be designated as such. This can be done in several ways, such as:

• Adding a rel=”nofollow” attribute to the <a> tag
• Redirecting the links to an intermediate page that is blocked from search engines with a robots.txt file”

Here follows an email conversation from today: Read more