I do have sole agency, and that cannot be sold, transferred, etc... Since property universally can be sold and transferred, that agency over myself is not proprietary, i.e. I am not property.
It is sovereignty, not property, that is pertinent to that aspect of me.
The similarity of the two words regarding agency is potentially confusing, but like air and water which both flow, sovereignty and ownership are intrinsically different. The big difference is that property can be sold, and sovereignty cannot.
I am not property because I am sovereign, and this means that slavery cannot be lawful under any just legal system or theory. Slavery is currently legal in the USA because of the missdefinition of people as their own property under the 13th Amendment to the Constitution, and so this is not merely pedantry, but a very meaningful difference.