[...] Google said that some sites that are selling links may indeed end up being dropped from its search engine or have penalties attached to prevent them from ranking well.
[...] Google says that most people hit with a PageRank decrease will likely notice this, and then they can request a review.
[...] Google stressed, by the way, that the current set of PageRank decreases is not assigned completely automatically; the majority of these decreases happened after a human review. That should help prevent false matches from happening so easily.
[...] Ironically, despite the move, Google itself will still allow paid links to be promoted in another way -- through its own ads.
Selling links can damage your PageRank or SERP position on Google
Seems that Google is definitively feed up of sites made only for pagerank selling. The keyword "buy pagerank" was been a lot penalized through the first weeks of October 2007, and the websites which were selling PR find out a level or two of decrease in their pagerank. And what with the SERP (Search engine results page) position? (Quotes from the article of Danny Sullivan (October 7, 2007 - source: http://searchengineland.com/071007-173841.php)