If I were in your position I would ask my professor the following hypothetical:
Say you are a very poor peasant, your standard of living is awful and your life expectancy is mid-thirties. On the other hand you can self yourself to the local lord and have a much higher standard of living and have a much greater life-expectancy.
Would he choose to sell himself or remain a free peasant? Even if he would choose to remain free, would he not understand why someone else would choose to sell himself? Is his moral problem with slavery a problem with the American institution or with the idea that people can own other people? If the peasant wants to sell himself because it benefits him to be a slave, could it possibly be immoral for the lord to purchase him (and treat him well)?
Agav, I suspect that much of the slavery in the Torah that he is familiar with will be eved ivri (which, in American law, would be indentured servitude, not slavery). You could then point out the great social advantages of the institution of eved ivri.