Who defines permitted contracts in a free market? Some right libertarians suggest that "free" markets include the "freedom" to sell labor by the lifetime or sell voting rights in the state.
"The comparable question about an individual is whether a free system will allow him to sell himself into slavery. I believe that it would." -- Robert Nozick
The theory that invalidates such contracts is the theory of inalienable rights. It has recently been shown to apply to capitalist employment