For the most part, slaves were sold to European traders by slave traders. These were people who bought and sold slaves just like any other commodity. But they were not just any slaves they got paid a little bit for there work
Most of the Chesapeake area slaves were sold because they were in the North before the Civil War and the North was supposedly against slavery. Many slaves were traded or sold if they were not performing as promised.
Slaves were taken by force , kidnapped or sold. Slaves were taken on large ships where they were treated brutally and merciless. Slaves were stacked on top of each other as if they were cargo, so more could fit, and more was sold.