That there is the difference between a conservative and a libertarian. Obviously, that slavery thing does not apply to apprenticeships, graduate students and the like. I don't see why a free and able person can't sell himself into slavery if he wants, or work for less than the minimum wage for that matter.
Hold on now. I don't know ANY conservatives that support minimum wage - So a bad example. Nor do I know any conservative that is against apprenticeships and the like.
Suppose a woman contractually sells herself to a master - A weirdo fetish, I know. As long as SHE wants to honor that contract, she IS a slave. But as it meets her actual liberty, the contract is null and void on its face. She cannot sell her liberty. It is endowed to her by her Creator, and is inalienable, according to every single word we are founded upon.
Contractual gymnastics are way different than the institution of slavery.
Even a case where the law requires recompense through labor - Indentured servant - would not ruffle me much. That again, is different from the institution of slavery.