People do have, in certain situations, the ability to sell their constitutional rights. You sell your right to free speech when you sign an NDA. You can't sell

People do have, in certain situations, the ability to sell their constitutional rights. You sell your right to free speech when you sign an NDA. You can't sell yourself into slavery because slavery itself is illegal. Slavery itself is also somewhat of a special case because entering into a contract requires the ability for each party to seek redress or other appropriate methods of enforcement. But once a slave, all personal agency is lost and therefore the enslaved party is no longer on equal grounds with respect to having the contract honored. It is an inherently paradoxical contract. Contrast this with selling your right to free speech with an NDA: you still have access to all the methods of enforcement and redress if disagreements about the terms arise while the contract is in effect.