Recently we have found, lots of sites lost their Page Rank in the current page rank update by Google. Experts say, sites which have lost their PR, maximum of them sell links.
But now the question arises how Google determines which sites sell links?
We can remember the article “Why should I report paid links to Google?” And in that article one link named “Report paid links” was live. People were asked by Google to report sites which sell or buy links. Thousands of sites had been reported by people. After having the list of sites reported by people Google probably asked some of his employees to check manually whether these sites involved in link selling or buying.
Here is a screen shot of the form of reporting paid links: