This year in January 2011, Google Webmaster Tools begin sending out "spam notifications" to webmaster including notifications that your site has been penalized because of selling links.
Now, Google is notifying webmasters about the buying links, reports SER.
In the email below, you can see, Google is telling webmasters that their site has been penalized because they have bad links, maybe paid links or other unnatural links, pointing to your site. And, the specific line "unnatural links pointing to your site" versus the old notification that read, "unnatural links on your site pointing to other sites."
Here is a sample of the email notification from Google:
Dear site owner or webmaster of http://www.domain.com/,
We've detected that some of your site's pages may be using techniques that are outside Google's Webmaster Guidelines.
Specifically, look for possibly artificial or unnatural links pointing to your site that could be intended to manipulate PageRank. Examples of unnatural linking could include buying links to pass PageRank or participating in link schemes.
We encourage you to make changes to your site so that it meets our quality guidelines. Once you've made these changes, please submit your site for reconsideration in Google's search results.
If you find unnatural links to your site that you are unable to control or remove, please provide the details in your reconsideration request.
If you have any questions about how to resolve this issue, please see our Webmaster Help Forum for support. Sincerely, Google Search Quality Team