Not every society that practiced slavery regarded your slave status as an immutable part of your being nor does slavery require trade if you are born a slave in a family and live in bondage you are a slave without having changed hands.

To be a slave is to have your personal freedoms subordinated to another while people always enjoy a finite degree of autonomy which varies substantially from society to society and even between different socioeconomic groups if your owner can tell you where to go and whom to kill, and in fact to surrender ones life if asked then I cannot discern the difference between that state and slavery.