In last week of Oct, Google begun its crackdown on sites, which were dealing in paid links in an effort to de-influence on their Google PageRank. Followed by, Matt Cutts, unofficial confirmation of this a couple of days after the shrinking Google PageRank across such sites.
Google has now made it official, by adding to its Google Webmaster Help Center that selling links that pass PageRank can penalize a site not only in its Google Toolbar PageRank status, but also in Google search results.
From the Google Webmaster Help Center:
Paid Links, Google, Google Webmaster
Old — “Buying links in order to improve a site’s ranking is in violation of Google’s webmaster guidelines and can negatively impact a site’s ranking in search results.”
Now — “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.”