There are a lot of website ranking systems available online and page rank should not be one of it. Alexa, Compete, Quantcast, etc measures ranking from traffic but page rank is from backlinks. It is totally different view and it should not be considered as ranking at all. We can get page rank juice from a dofollow backlink. If the page having high PR, low outbound links, relevant info, then the passing value or PR juice will be heavier. If an administrator owns hundreds of website or blogs that is hosted on separate hosting IP and domain names. He can manipulate pagerank value for a particular website by giving a backlink from hundreds of high pagerank websites with unique IP and domain name.
It may lead that particular website to get good pagerank and it may affect the value of re-selling. Unlike other ranking system, pagerank becoming more visible these days with some tips from Matt Cutts and other SEO experts. This may lead to easy manipulation of pagerank value to any website, thus giving almost no real value. The visibility of pagerank for each page especially higher PR, makes it vulnerable for linkbaits, backlink creation, etc. This motivates link buyers to get only dofollow links which is against Google TOS and may lead to PR penalty for the sellers.
It may be difficult to monitor billions of websites on paid links with dofollow value. So, I can only see one good solution that stops buyers requesting dofollow links. Hiding pagerank value or removing it from public may be the only solution. Pagerank should be internal ranking system in Google and should not be shown to the public. In my opinion, showing pagerank will only lead to more dofollow link sales and purchase in online industry. It may lead to penalty to those innocent sellers who are not aware of dofollow and nofollow attributes or Google TOS.