But "as most would define it" is precisely part of the debate. Remember, part of the problem here is defending Scripture's acceptance of, if not the outright condoning, of slavery. And part of the debate must necessarily be that slavery as Scripture defines it is different than how most would define it today.
Under the Scriptural definition of slavery, voluntary slavery is explicitly mentioned in Exodus 21:2-6. This passage commands Hebrew slaves to be set free in the Sabbath year. But such a slave could volunteer to remain with his master; if he did so, his ear was pierced to indicate this commitment. Many translations use the term "servant" in this passage, but it is clear the person in question is in fact a slave, for two reasons: first, he is purchased, not employed; and second, if he volunteers to stay, he must stay for life.
So again, when someone wishes to argue that Scripture condones slavery, I think that it is important to distinguish between our understanding of slavery, motivated by our country's history, and Scripture's understanding of it.
We agree 100% on the particulars of the Western slave trade, and as I said I appreciate your approach justifying a broad condemnation of slavery under the New Covenant.