But there was a domestic slave trade, as well. The international slave trade was forbidden in the US during the Jefferson Administration. But the domestic trade lasted as long as slavery did. And it was an even more visible sign to free citizens of the ugliness of the slave system. Even planters regarded slave traders themselves as undirable characters. Which, of course, did not lead them to refrain from utilizing their services.
J. William Harris writes in "Eugene Genovese's Old South: A Review Essay" Journal of Southern History 80:2 (May 2014) about the slave trade in the quotation below. He was explaining why Eugene Genovese's interpretation of the slave system as fundamentally a paternalistic one failed to win the agreement of so many historians. Genovese's view was that the slaveowners' paternalism involved seeing the slaves in a protective way as part of an extended household.
Studies of the slave trade have perhaps done more than any other area of research to undermine the paternalist interpretation. Research by Michael Tadman showed that, between 1820 and 1860, nearly 900,000 slaves were taken from southeastern states to those in the Old Southwest, over half through the interstate trading system. Tadman estimated that over half of all interstate sales either split husbands from wives or took children under age fifteen away from their parents. Walter Johnson's cultural history of the trade argued that slaves and slaveholders held "radically incommensurable views" of the relationship of the trade to slavery itself; for slaves it was the trade, more than the plantation, that bared the essence of a system that made the slave into a commodity, "a person with a price." Paternalism, Johnson argued, "was a way of imagining, describing, and justifying slavery rather than a direct reflection of underlying social relations"; paternalism could, indeed, be "something slaveholders could buy in the slave market," based on a fantasy that they were doing slaves a favor by purchasing them. In a separate essay, Johnson noted that many planters' expressions of paternalism appeared in comments on the deaths of favorite slaves or in postwar memoirs. Such paternalism, he wrote, "as often expressed a sort of nostalgia for dead slaves and the lost cause as it did the actively governing ideology of a ruling class" and thus "seems more properly read as a sort of a pose that slaveholders put on for one another than as a praxis through which they governed their slaves." [my emphasis]