The forcible enslavement of human beings was permissible and enforced by law in multiple states until 1865. Although you could quibble about whether the seceding states counted as part of the US from 1860-1865, slavery remained legal in Maryland until the passage of its 1864 Constitution, and it remained legal in Kentucky until the passage of the 13th Amendment. There were also some states, like New Jersey, in which the acts of enslaving a person and importing an enslaved person were illegal but the manumission of existing slaves was not required.
And it wasn't just chattel slavery. A whole lot of the non-chattel laboring class consisted of indentured workers and debt peons, as well.
But, as David Graeber put it, that's the point. The people who invented the anarcho-capitalist philosophy of self-ownership, or natural ownership of one's "liberty", mainly did so because they wanted to explain how one could trade away that very liberty for money.
