From its early days and first implementation of Page Rank algorithm, Google forbade selling links. After all, if people link to your content without monetary incentive, it has to mean that it is useful or important to them in some way.
In those early days, first paid SEO services showed up. Most of them had no idea what they were doing but the Dot Com bust was just starting. A lot of money ended up wasted on some useless linking services and soon, Google coined the term “link-farm” and first Google slaps ensued.
Latest Google slap of sites using one paid SEO service shows how expensive SEO shortcuts can be.