Slavery has existed from the start of human civilization. When a kingdom was conquered by another, the subjugated population often had two options death or slavery as imprisoning large numbers of people was not feasible or possible. Slavery also often existed as punishment for a crime, as a means of repaying an exceedingly-large debt, or a way to earn something very valuable. All these forms of slavery were lawful according to the Bible. However, the Bible does not permit kidnapping someone then selling them into slavery as was done in the transatlantic slave trade: “If someone is caught kidnapping a fellow Israelite and treating or selling them as a slave, the kidnapper must die. You must purge the evil from among you.” (Deuteronomy 24:7.)
There is an important caveat here. A large number of African slaves sold to European powers were prisoners of war by rival African tribes. Slavery was permitted in this case. However, a large percentage of African slaves were not POW’s and this form of slavery is illicit. And one might argue if the purpose of war in which African tribes would attack each other was for the sake of acquiring slaves to sell to the white man, this could be seen also as a violation of Deuteronomy 24:7 and a large percentage of these POW’s were likely captured this way, I suspect.