When you’ve been blogging for awhile, you’ll undoubtedly be approached by PR people wanting to place an ad or link on your website for a fee. What is important to realize is that links going to their site are good for their search engine visibility. And that is fine. However, Google frowns (really gets quite cranky) about non-organic (sold) links that impact their results. So the idea here – always apply ‘no-follow’ on ads you sell. To do otherwise, could mean a penalty from Google.
And for more on this subject, visit the expert – Yoast.com: Link Selling Stupidity. If you are paid to link to another site or page or product, you MUST read this post on using no-follow, or face the consequences!