Final answer:
The domestic slave trade involved the selling of enslaved people within the United States, with slaveholders in the Upper South selling individuals to states in the Lower South. This trade caused the forced separation of families and resulted in immense suffering.
Step-by-step explanation:
The domestic slave trade functioned by selling enslaved people within the United States. Slaveholders in the Upper South sold enslaved individuals to states in the Lower South, where the demand for labor was increasing due to the growth of cotton and sugar economies. The trade was conducted through slave auctions and involved the forced separation of families, causing immense suffering and heartbreak.