"It is not simply that the labor of enslaved people underwrote 19th-century capitalism.
"Enslaved people were the capital: four million people worth at least $3 billion in 1860, which was more than all the capital invested in railroads and factories in the United States combined.
"Seen in this light, the conventional distinction between slavery and capitalism fades into meaninglessness."
You are conflating "commerce" with "capitalism"...selling a person who is a slave doesn't mean you are practising capitalism...since one party is not free to give his consent to the transaction...
Commerce is the exchange of commodities, and capitalism treats labor as just another commodity. You may not like it, but this country doesn't exist today without genocide and slavery.
Capitalism is free exchange of goods and services to the mutual profit of all trading parties. Note that this includes the exchange of service (labor). I assure you it exists as I personally engaged in it, this very day.
I spent an hour engaging my extensive experience, knowledge and tools... which solved a problem for an individual who knew of my skills in resolving such in a timely and professional manner. For that hour of time, I received a couple of hundred bucks.
They were happy, having had their problem resolved, thus representing their profit and I was happy having increased my means to fulfill my own life and that of my family.
Works every single time it is exercised by reasonable people, intent on bearing the responsibilities that sustain their right to exercise their right to do so.
Simple stuff... despite being beyond the means of the intellectually less fortunate.