Frederick Douglass's thoughts about the internal slave trade are not found in the book you mention. They are found in a speech he gave on the Fourth of July, 1852 in Rochester, New York.
In that speech, he says that the thing that is even worse than the internal slave trade is the Fugitive Slave Act of 1850. That law made it very easy for any black person in the North (whether an escaped slave or not) to be taken and sent into slavery. Douglass says this law makes the whole of the country a slave country by forcing the Northern states to help send slaves back into slavery.