In the United States, there is no entity or heir to hold responsible. It was the US government that ended the practice, at no small cost of lives and fortune. There is the added consideration of the individuals. Most families, even most southern families, never held slaves. Reliable statistics demonstrate fewer than five percent of the population were ever slaveholders.
Fortunes have been lost and changed hands, the people,circumstances and attitudes are different.
Recent legal wrangling provides a more elegant solution. No, manufacturers have been the targets. Surely the slave traders should bear responsibility. The full force of the US government making a legal claim, on behalf of her citizenry, regarding the moral principle of slavery might force Africa to finally address its slave-holding ways. If reparations are to be sought, let us seek them from the source.