Douglass does not talk about this in his autobiography. Instead, he talks about it in the speech he gave in Rochester, New York on the Fourth of July 1852.
In this speech, he says that the American view of the internal slave trade is hypocritical because Americans think that it is in some way different than the external slave trade.
He points out that people say bringing new slaves from Africa is horrible and that the US spends a lot of money preventing this trade. But, he says, there's no difference between that and selling slaves within the US.
So he says it's hypocritcal to criticize one kind of slave trade and condone another.