Slavers like pirates and bandits made a living by taking things that weren't theirs however unlike pirates who stole items slavers stole lives. Much like bounty hunting and assassination slavery was a major industry in many portions of the known galaxy. Due to the fact that slavery was outlawed by Republic law the majority of this deplorable activity occurred in the less policed Outer Rim territories. The Hutts for example became rich selling people and weapons on planets like Tatooine. On Ryloth innocent Twi'leks have been hunted and captured to be sold across the galaxy for centuries. In fact on Ryloth slavery is so pervasive the government has all but made it legal and even the citizens of the Republic, well aware of the the conditions on the planet, are familiar and do tolerate the sale of Twi'lek dancers in their space.
Another of the most well known slaver organizations is the Zygerrian Slavers Guild. During the Clone Wars, they allied with the Confederacy of Independent Systems and used this alliance as a means of acquiring the Togruta colonists of the planet Kiros. Working with Count Dooku the Zygerrians whose cultural seems to venerate the master and slave dichotomy believe that the CIS will be able to help them rebuild their empire on the backs of all free sentient life in the galaxy.