lesser & inferior human beings? yes. But still human.
therefore, even back then, one could not be both a slave-owner and a Libertarian.
nor could one be a Libertarian and believe women or Jews should not have the right to vote.
if you believe someone is a human being in any form, no true Libertarian would accept that person being denied any rights or freedoms.
Voluntary slavery (or self-sale) is the condition of slavery freely entered into. In ancient times this was a common way for impoverished people to provide subsistence for themselves or their family and provision was made for this in law. For example, the code of Hammurabi stated that, "besides being able to borrow on personal security, an individual might sell himself or a family member into slavery."
In ancient times one of the most direct ways to become a Roman or Greek citizen was by means of a self-sale contract. For the laws surrounding Roman and Greek manumission made it quite possible for such erstwhile slaves to then become citizens or near-citizens themselves.
In medieval Russia, self-sale was the main source of slaves.
1: M. L. Bush (1996), Serfdom and slavery: studies in legal bondage, p. 21
2: Anthony Appiah, Martin Bunzl (2007-07-02), Buying freedom, pp. 95–97, ISBN 978-0-691-13010-1
3: The International Standard Bible Encyclopedia. Geoffrey W. Bromiley. Wm. B. Eerdmans Publishing, 1995. ISBN 0-8028-3784-0. p.543.
4: MA Klein (1986), "Slavery in Russia, 1450-1725", American Journal of Sociology
“And I have no doubt that every new example will succeed, as every past one has done, in shewing that religion & Govt will both exist in greater purity, the less they are mixed together.”
~ James Madison, letter to Edward Livingston, July 10, 1822