Libertarians believe that self-ownership is immutable and that natural law and inalienable rights deem it impossible for another person or entity to own human beings.
Beyond ownership, libertarians adhere to the principle that an individual maintains bodily autonomy and may do whatever they desire to their own person at their own jurisdiction and justification.
Though slavery still exists in the modern world, libertarians maintain that the actual possession of another person is inconceivable. The transaction of self-ownership to outside ownership is not tangibly executable, regardless of what is decreed by the state, governing authorities or other agencies.
A human being retains their sovereignty--philosophically speaking--even under forced labor, incarceration, conservatorships, military service etc.
Libertarians hold self-ownership as a core value. To dispute the concept and paradigm of self-ownership is a blatant contradiction to the philosophy of libertarianism.
"Every man has a property in his own person. This nobody has any right to but himself" ~ John Locke
For more information, see 'What is Libertarianism?'