People have sold themselves throughout all the history of mankind. Into slavery, into safety, into marriage, into religion for immortality, on and on.

What!!!!

Self ownership is entirely necessary in order to morally prevent others from asserting that they or it or whatever owns you and that it's right for you to be responsible to demands placed on you because of that ownership, even to self sacrifice. If you fail to assert your ownership of self, others will.

On a gramatical note, English makes us use reflexive pronouns when the subject and object of a verb are the same, e.g. "I own myself." instead of "I own me." (except in cases of grammatical license as in the OP) Maybe this is because something verbs by their nature are different when the subject and object are the same. This is how I see "to own".

My understanding is we do own ourselves. Owning oneself, IMHO, is different from owning anything else. As Salty says, you can't permanently sell yourself, relinquishing all ownership. This is because permanently selling yourself would mean splitting the part of you with agency from some other part, which is impossible. That doesn't mean, to my understanding, that we do not own ourselves.

If you do not take ownership of yourself, how do you contract? It is the most fundamental property right.

I think that first of all, before analyzing the use of own logically, you should first define your term 'own'. If you mean it in the sense of possession, your premise is invalid. If I truly own something, I have the right and ability to, sell, give away or destroy the article. I can sell my car, I can give someone a guitar and I can burn my newspaper if I so choose. None of these apply in the case of a human being. In that sense, the concept of belonging does not refer to ownership, but rather to membership. I'm a member of the band; they, however, don't own me in any real sense.