Well, I think you basically answered the question right there. Everyone who has a job is essentially "selling" him or herself and their abilities, talents and physical labor. The key difference is that they are doing so for their own self-interests, not through by any involuntary measures.
Since there are already laws that prohibit the "owning" of other human beings in bondage, one could technically offer him or herself up for sale as a slave, I suppose, but then, no one could "buy" them either.
It's a superficial answer and does not bear up to scrutiny. A follow-on question could be along the lines of what part of you is 'you'.