It was mostly Arab slavers, whites traveled months across treacherous seas to get to Africa, and months to get back. It made no sense to risk the lives of your crew, hunting down potential slaves in a land and climate foreign to you, against armed and hostile natives. This could take months. Supplies would have to be brought with you, you wouldn't have time to gather them and hunt slaves at the same time. As you accumulated slaves, more and more of your crew would be required to guard the slaves, leaving fewer and fewer to hunt down the hostile natives. If too many of the crew are lost due to fighting the natives, or disease, you wouldn't be able to sail home. And there was a ready supply of already captured and enslaved victims, at very reasonable prices. You wouldn't go to China and raise your own silk worms and make your own silk, to take back to your homeland would you? No. you'd just buy or trade for it from the locals. Rival tribes would sell conquered enemies into slavery and take their land, etc. Sometimes, local chieftains would sell "excess" population, or possible rebels or rivals, or just a nagging wife. The Kennedy family made their fortune in the slave trade, a little bit of American history that has been hidden.