How to Remain Safe?
You are the webmaster of your site, it is your responsibility to follow the guidelines of Google and never indulge in spammy activities of buying and selling links. Selling links might give you temporary profits but in the long run its going to adversely affect your website.
Have you received a message of unnatural links in your webmasters account?
If you have recently received a message of unnatural links pointing to other websites with the sole intention of passing PageRank then be cautious and take instant action. Make sure to make all the links to "no follow" and sent a reconsideration request to Google that you have taken sufficient action against the links and have added the rel no follow attribute to the unnatural OBL's to stop the passing of PageRank. Make sure to follow the Google's Webmaster guidelines.