Right out of the Google Bible, under the Quality Guidelines section:
Don't participate in link schemes designed to increase your site's ranking or PageRank. In particular, avoid links to web spammers or "bad neighborhoods" on the web as your own ranking may be affected adversely by those links.
So why do we see ads served up by Google's AdWords program, with content in the language of "Buy High PR Links:" "We have a huge selection of high ranking relevant sites" asks one member at WebmasterWorld. In fact this question has been asked several dozen times at forums all over this industry. Here is a snap shot of what one ad looks like (it is not clickable):
But at WebmasterWorld, a Google AdWords representative, under the username "AdWordsAdvisor" gives the official Google response:
So, advertising to assist with PageRank isn't a problem. Selling PageRank, however, is another thing - and ads/sites are reviewed on a case-by-case basis along these guidelines.
If you're seeing ads promoting the sale of PageRank, then it's possible that those ads have not yet been reviewed/disapproved. At least let's hope that's it. ;)
He then is asked for further clarification:
* Ok to assist folks in understanding PageRank - and how to improve it.
* Not OK to sell it, or sell it while calling it something else.