Slaves were offered for sale in Africa by Africans.
If slaves weren’t offered for sale in Africa, sailing ships manned by white people would not have been able to bring them over in massive numbers to badly infect the New World with slavery.
Not to mention that the indigenous empires of central & south America fought wars against neighboring tribes in large part to enslave their peoples. So it’s note even a unique characteristic of white Europeans to act the part of slavers. Every race did it for most of human history for varied reasons.
At the time that the Spanish, Portuguese, French, Dutch and English were bringing Africans to the New World to work as slaves, Europeans were being enslaved by the Turks. Some of my relatives were captured by Turkish pirates in 1715 and spent the rest of their lives as slaves.
When the US declared its independence, slavery was already well-established in the colonies. Even though some of the Founding Fathers recognized it was an evil institution, getting rid of it was not easy. You couldn't just go to the court house and sign a piece of paper. States did not want a lot of indigent former slaves who might be forced to steal to stay alive. George Washington decided to free his slaves and it turned out to be very complicated because he had to figure out a way for them to make a living afterwards, and because he did not want to break up families (some of the slaves at Mount Vernon were his wife's dower slaves and he had no authority to free them--and some of them were married to his slaves).