Webmaster level: all
Google has said for years that selling links that pass PageRank violates our quality guidelines. We continue to reiterate that guidance periodically to help remind site owners and webmasters of that policy.
Please be wary if someone approaches you and wants to pay you for links or "advertorial" pages on your site that pass PageRank. Selling links (or entire advertorial pages with embedded links) that pass PageRank violates our quality guidelines, and Google does take action on such violations. The consequences for a linkselling site start with losing trust in Google's search results, as well as reduction of the site's visible PageRank in the Google Toolbar. The consequences can also include lower rankings for that site in Google's search results.
If you receive a warning for selling links that pass PageRank in Google's Webmaster Tools, you'll see a notification message to look for "possibly artificial or unnatural links on your site pointing to other sites that could be intended to manipulate PageRank." That's an indication that your site has lost trust in Google's index.
To address the issue, make sure that any paid links on your site don't pass PageRank. You can remove any paid links or advertorial pages, or make sure that any paid hyperlinks have the rel="nofollow" attribute. After ensuring that no paid links on your site pass PageRank, you can submit a reconsideration request and if you had a manual webspam action on your site, someone at Google will review the request. After the request has been reviewed, you'll get a notification back about whether the reconsideration request was granted or not.
We do take this issue very seriously, so we recommend you avoid selling (and buying) links that pass PageRank in order to prevent loss of trust, lower PageRank in the Google Toolbar, lower rankings, or in an extreme case, removal from Google's search results.