In addition to the other answers, consider slavery in the distant past. In ancient times, slavery was more like indentured servanthood in some cultures. That meant that slaves were only "temporary" and that once they paid off their debt, or a family debt, they were freed. This is evident in the Bible where there are many instructions for slaves to obey their masters. Slavery was not the same as in later times. Slaves were more like servants.
Many of the African cultures understood slavery to be only temporary, so when they at first sold off their own slaves, prisoners from other tribes usually captured in war (usually but not always), they expected that it would only be temporary. They did not expect that the slaves would be thrown onto a slave ship and brought to another country, oceans away, never to be seen again and never to be free again.
There are various types of slavery as well, even today. In some countries in the former Soviet Union, women and men alike are used as sex slaves. This is also true in some Asian countries. Plus, in places like Darfur and other oppressed African nations, people from other tribes and other religions are being enslaved by opposing groups and outlaw groups.