Several decades ago, slave trading was conducted more openly than in the present. Humans were greedy creatures, willing to do anything to get what they desired, which included selling others’ lives to obtain money. There were several methods they used to obtain slaves, most of which were in conflict with the moral standards that humans themselves liked to preach about.
Sometimes, the slave traders would just outright capture people and turn them into slaves. Other times, they would coerce them into becoming one. The life of a slave was like a living hell. Some slaves may have it better than others if they had a kind master, but some were more unfortunate and forced to work like animals.