They certainly sold them -- often to Muslims.

Slavery has had a very long history in human affairs. Bartering in people was a norm (although people tended to not barter their own people -- but there would be no restrictions on people outside the tribal group). And, it hasn't always been of the type we think of today in the southern American states. We could even make the argument that serfdom was a form of slavery -- there was a serf class in many societies that was based on family lineage.

We should all perhaps quit worrying the bone of "who enslaved who" one hundred years ago (everyone was pretty much guilty); all agree that it is an ugly practice that we should eradicate; and be thankful we aren't ok with it as much as we used to be (but perhaps a little too comfortable with it still).