Yesterday, both Matt Cutts and John Mueller of Google answered the same question on PageRank penalties for selling links without knowing the other answered them. It has been a while since a Googler talked about selling links and the penalty associated with it, so to have two Googlers address it the same day is a special day.
Matt Cutts posted about it on his personal blog, titled why did my pagrank go down. In short, the site was selling links that passed PageRank and thus Google downgraded the PageRank of the site and trusts the links less on the site. This is nothing new, covered it several times.
John Mueller answered it also on a Google Hangout, about 40 minutes 32 seconds in on this video. I'll embed it below and you can see he calls me out a bit.
Before anyone starts yelling their heads off at me, do so on this post and not here.