That's a complex question. It's hard for us to judge isn't it?! Slaves can be defined in modern terms as, "cheap labor". Slaves like Vixen said came from Africa, but were sold by white people, mainly.
In that century Europe and China were the major, hubs of wealth and industry, they were more sophisticated and technologically evolved. They had guns, canons, weapons that were far superior to those of the African's. And as the saying goes "Don't bring a knife to a gun fight!" or in this case spears to a fight. So the people of Africa were literally forced to give up.
The slaves that worked in America were there to literally do the dirty work, the work the founders didn't want to do. One must also remember when America was first colonized, there were many wealthy amongst the first settlers and they could afford to buy labor.
So to really answer your question, I'm sure some of them must have been, but take into consideration slavery still exists in today's world, just under a different name.