... in the ancient world there were many “slaveries.” There is good evidence that much of slavery was very harsh and brutal, but there is also lots of evidence that many slaves were not treated like African slaves would be, but lived normal lives and were paid the going wage, but were not allowed to quit or change employers, and were in slavery an average of ten years.
 
Prisoners of war often became slaves, and men could be sentenced to being galley slaves for crimes. A person could become a slave for a set period of time in order to work off debts, because there was no such thing as bankruptcy in ancient times. Often the result was an indentured servanthood for years until the debts were paid.
 
To our surprise, slaves could own slaves, and many slaves were doctors, professors, administrators, and civil servants. (See Andrew T. Lincoln’s discussion of ancient slavery in his Word commentary on Ephesians, 415–20.) In his survey, Lincoln says that no one in ancient times could conceive of an economic or labor structure without it. While there were brutal forms of slavery, the concept—indentured labor in which the laborer was not free to market his skills to other employers—was considered a given. Quoting another scholar, he writes that this was so accepted, “one cannot correctly speak of the slave ‘problem’ in antiquity” (Lincoln quoting Westerman, 415.) In other words, no one—not even slaves—thought the whole institution should be abolished.

 
And this is from Carson:

Please understand me: I’m not trying to romanticize slavery in any way. However, in Roman times there were menial laborers who were slaves, and there were also others who were the equivalent of distinguished Ph.D.’s, who were teaching families. And there was no association of a particular race with slavery.
 
In American slavery, though, all blacks and only blacks were slaves. That was one of the peculiar horrors of it, and it generated an unfair sense of black inferiority that many of us continue to fight to this day.
 
Now let’s look at the Bible. In Jewish society, under the Law everyone was to be freed every Jubilee. In other words, there was a slavery liberation every seventh year. Whether or not things actually worked out that way, this was nevertheless what God said, and this was the framework in which Jesus was brought up.
 
But you have to keep your eye on Jesus’ mission. Essentially, he did not come to overturn the Roman economic system, which included slavery. He came to free men and women from their sins. And here’s my point: what his message does is transform people so they begin to love God with all their heart, soul, mind, and strength and to love their neighbor as themselves. Naturally, that has an impact on the idea of slavery.

 
The whole piece is well worth a read.