Was slave trading allowed in early islam?

I know that today there is a general consensus that slavery is banned. And that the only way to get slaves are by taking prisoners of war and giving them good treatment and the option to free themselves once they collect a certain amount of money. My question is that was trading slaves allowed in early muslim society ? Were people able to buy and sell slaves to one another ? I couldn't understand this because it seems to compare slaves as objects or animals, please clarify this for me :) jazakum allahu khayran