As soon as some folks realized that links on external sites could drive up your Google rank, the SEO industry exploded in growth. It was a billion dollar market and Google quickly lost control of providing users with great results. It turned into a contest of who paid for the most backlinks. Thankfully, for deserving marketers, these SEO cheaters have been largely halted. Google algorithm changes have unraveled poorly placed links and they've even started penalizing companies where they're finding unnatural links.
Links puchased with the intent of altering or manipulating search results are in violation of Google's Webmaster Guidelines.
And don't believe anyone that says that you can purchase content with links and that somehow doesn't violate the terms. If the goal of the purchased content was to place that link, you're still in violation of Google's Terms of Service!
This infographic, Tactical Touchdowns – The Link Acquisition Playbook, is a great infographic that spells out the challenges, the guidelines, and the opportunities to build links naturally. The answer, of course, is really to forget links at all and just focus your efforts on developing incredible content.
Great content is shared. Sharing is done in relevant networks. Relevant sharing drives relevant links. Relevant links drive rank.