According to Matt Cutts of Google, PageRanks go down when selling links occurs. The links are sold to pass PageRank, which is basically like buying a review.
Google considers it unethical and in some cases, dangerous. An example of the danger arises from medical issues, like a writer endorsing a medical procedure they know nothing about and have not bothered to research. All they do is make it sound good.
PageRank violation is one of the more common reasons why businesses lose their online clout, but it can be rectified by removing the links. After removing the links, a business should reach out to Google to show them that the issue has been amended.