LyricalX:
What you have to understand is that this book was written just shortly after the Israelites came out of hundreds of years of slavery and thus, they were used of being ordered around to do this and to do that, and not to do this and not to do that. The whole book of Leviticus is basically what God expects of His people, whom He brought out of slavery. If I could have, I would recommend that you skip that particular book as it doesn't have an overly large significance on Christians.
Also, if you read other parts of that same passage, you would see that God said: "If a fellow Israelite living near you becomes so poor that he sells himself to you as a slave, you shall not make him do the work of a slave. He shall stay with you as a hired man and serve you until the next Year of Restoration." Why? No man can serve two masters. God justifies this with: "The people of Israel are the LORD's slaves, and He brought them out of Egypt; they must not be sold into slavery."
So, what if you need people to work for you but can't afford to hire Israelites, then God said that, "You may
buy them from the nations around you." But, God makes provisions to ensure that they are treated well, in Leviticus 19:18 - for example - we read: "Do not take revenge on anyone or continue to hate him, but love your neighbour as you love yourself." And in Leviticus 19:33-34 God commands the Israelties: "Do not mistreat foreigners who are living in your land. Treat them as you would a fellow Israelite, and love them as you love yourselves."
I view that slaves are apart of the foreigners in the land and come under that definition and they are to be treated well. This was also a command for the Israelite people.
You must also consider the commands that Paul gave Christian slave owners at that time, for example, in Colossians 3:22-4:1 in which Paul says:
"Slaves, obey your humans masters in all things, not only when they are watching you because you want to gain their approval; but do it with a sincere heart because of your reverence for the LORD. Whatever you do, work at it with all your heart, as though you were working for the LORD and not for men. Remember that the LORD will give you as a reward what He has kept for His people. For Christ is the real Master you serve. And every wrongdoer will be repaid for the wrong things he does, because God judges everyone by the same standard. Masters, be fair and just in the way you treat your slaves. Remember that you too have a Master in heaven."
He also reiterates this idea in Ephesians 6:5-9.
You see, in Paul's day, slavery was a common occurance and an every day thing. Thus, like with other things (e.g. the roles of women), the early Christians worked within the framework of society rather than radically changing it to fit with a Christian world view in which all men and women are equal, because doing so would have made it extremely difficult for the Apostles and the early Church to preach and spread the Gospel - probably because people would have been scared away by their radial teachings and the focus would have been turned away from Christ.
You'll notice as you read the letters written by Paul that he brings Christian morality and ethics into these situations (e.g. slavery) that are considered common and right in his day. It is my conviction that those so-called "Christians" who were enslaving the people of Africa were being inconsistent with their faith.
I hope this helped you a little. If you have any more questions, don't hesitate to ask.