Every contract is not valid and no contract should ever be enforced by government. That's not the point of contracts.
And most certainly people should be able to sell their organs. You prefer the organ market fully socialistic as it is today? Some bureaucrat deciding who lives and dies? FFS.
And you can't legitimately sell yourself into slavery because you cannot sell your right to seek freedom. It's an unalienable right to act to seek freedom. You would need a state to enforce slavery, as slavery has always been a creation of the state.
You cannot sell your moral authority to act for your liberty because you do not own that authority. It came from your creator. It is unalienable, whether you believe in self ownership or a deity owns you, no other [i]man[/i] can ever own your moral right to act.