The recent PR update is very clear in pulling-out the PR of websites which is getting money and passing PR. As per the new webmaster guidelines, Google is telling that selling links and passing your PR for SE rankings is not recommended and not legal. So webmasters, please do not sell links and get caught by Google.
So, if any one is able to find such websites, please help Google to find the illegal website. Click here to register a complaint about link selling website.
Please note, you can always put ads / sponsored links in your website, but add a "nofollow" tag or add a intermediate page which will redirect to destination page. But this redirection should be blocked for search engines, you can use robots.txt to do this.