I've run into people who deny this but also read The Classic Slave Narratives by H. L. Gates where Olaudah Equiano describes this very thing.
But I don't argue. I'm not an argumentative person usually. Should I?
Olaudah's is such a great and fascinating story. He describes how by luck and pluck he obtained relatively decent treatment for himself but lived in the knowledge that he could be sold to, say, a Haitian sugar plantation. He had no real control over that so it was a constant underlying fear.