Since links with specific anchor text are highly valuable in SERPs and SEO, it was only natural that webmasters with good Pagerank and good SERPs would sell optimized back-links for a one-time or monthly fee. Not only would the purchaser of the links get click-through traffic, they could expect to see a jump in the search engine results. This practice is perhaps as old as the internet itself, but its fading out of existence.
What was once a no-catch source of practically guaranteed income has become a huge liability. Google has made it explicitly clear that a link from your website should be a vote of confidence, not for sale to the highest bidder. If you want to sell links, that’s fine: Just use the “rel=nofollow” attribute so the search engine spiders understand that the purpose of your link is self-serving or if its something you don’t necessarily want to take responsibility for (like perhaps comments in an unregulated comment system).
If you don’t like Google’s rules, that’s fine. You own your own website. Thing is, they own their website and can do whatever they want, too. If you want traffic you’ll probably want to rank well on it – so there is plenty of incentive for website builders to play along. Some people have found their websites dropping to zero pagerank overnight – some others don’t even care because they have other sources of traffic.
Some affiliate websites got caught up in this change and found their traffic plummeting. Adding no follow seems to fix this, or prevent the penalty. Then again, while Google might make its general intent clear, it is quite secretive of explicit algorithms. As a result, there is always a great guessing game being played. Guess right, and you win a lot of traffic. How much money you can turn that traffic into is up to you.