Everyone should understand one thing clearly, that page rank cannot be bought or sold. If anyone involve in this activity, the giant and the owner google will penalty your url to zero. There is only one legal way to earn page rank which does not involve buying and selling links with dofollow.
I have been experimenting two blogs, one under mixed bag category and another one in blogging resources. We all know that blogs with page rank that is passing page rank can get easy and higher ads rate. So far, only SocialSpark practices NOFOLLOW tags for paid posts. Other than that, BuyBlogReviews and SponsoredReview have some restrictions on NOFOLLOW, depending on the advertiser’s requirements.
Writing paid post with NOFOLLOW tags is the safest way for bloggers who wants to earn from paid posts continuously. This will protect bloggers hard earned page rank and gives more future advertisement opportunities while gradually increasing page rank.