Not is if it's "initially" entered into voluntarily.
If a desperate crack addict sold himself or herself into slavery for another fix, no American court would enforce the contract or allow its enforcement. Under Libertarianism, no 13th Amendment limitations on contractual agreements would be permissible.
Such slaves could be sold at auction like livestock and whipped into bloody pulps for attempting to escape, all in the name of contract.
What a loathsome philosophy.