Why did the europeans trade in slaves?

Europeans traded in slaves because this was a free source of labor. They did not need to do the strenuous work by making their slaves do it for them, so it was on a high demand. If you had to clear 100 acres of forest land, and one acre clearing would take 15 to 20 years, would you rather do it yourself or have someone else do it? I would go with the latter. Slaves were first indentured servants and then became slaves. I would tell you the whole story, but that would dwell on a different question.