This exact argument can be made against laws banning murder. The murderer can argue that he never signed a contract not to kill anyone. Thus, this argument ALONE is not sufficient.
Murder is covered under the non-aggression principle. There are no contracts needed for that.
WHY does the non-aggression principle apply? And why is intellectual work supposedly not protected by this principle?
This exact argument can be made against third party resellers of items acquired by theft. He never signed a contract with the original party and should therefore be allowed to keep the stolen goods. Thus, this argument ALONE is not sufficient. Illegal organ sales for instance, should by this logic be completely legal.
There are only two legitimate ways to obtain property, by claiming unowned property or by buying it from the
current owner. A person that receives stolen property from a third party has done neither so they don't legitimately own it.
Why does a person own his own body? He neither bought owned property nor claimed unowned property. I would claim that it is because he as an individual continuously mixes his WORK with the natural state and thereby acquire property rights. Simply "claiming" unowned property is not enough. I "claim" to own the entire universe that is as of yet unclaimed (i.e. the vast majority of it). Why is this not sufficient?
I own myself and therefore only I can legitimately sell myself into slavery, which I see no problem with.
WHY do you own yourself?
Let's recap, shall we. You think that you own something because you mix your labor with it.
Peacefully, yes. I.e. mixing labor with stolen goods or with slaves does not give rise to property.
I said that's false and I provided an example illustrating why.
You gave an example that I agreed with. You gave an example of work that involves violating others and hence is illegitimate. We don't disagree on that, so you can't use it as an argument against the labor theory.
Then you said (I think) that you can mix your labor with a book and thereby own the book. No, either you already owned the book and writing in it is just doing whatever you want with your property or someone else owns the book and you just defaced their property. Writing in a book has nothing to do with owning the book. Of course, what exactly that has to do with owning what's written in the book is beyond me but it sounds like question-begging thus far.
Here you are explicitly denying the existence of information, and then we are back to why information should be respected in other rights such as in contracts or when threats are made. As could use your argument above and say that I was not really making a threat. I was just peacefully waving my gun around while making a noise with my mouth: "give me all your money or I'll kill you!" If you perceived that as a threat then that's your problem. The same argument applies to contracts. If you deny the existence of information and only focus on physical actions, then I didn't sign a contract, I only write doodles on a piece of paper. Why is it ok to ignore the information content of a book, but not the information content of a contract or a threat?