How does Islam permit people to own, sell, and buy—including those who have embraced Islam—despite its call for equality and honoring humanity?

Islam did not invent slavery; rather, all previous religions and doctrines affirmed it and did not condemn it. Islam limited the causes of slavery to a single cause, which is jihad. In return, it opened many doors for freeing slaves and legislated human rights for them that did not exist before, such as commanding justice and kindness towards them, forbidding striking, humiliating, or torturing them, and prohibiting reviling them. Instead, it made them brothers to their masters. Islam did not abolish the system of slavery because it is a necessity dictated by wars and conflicts. Its abolition would mean killing prisoners of war, and slavery is a lesser punishment than death.