Way over emphasized by the neo-confederate propagandists.
Before the American Revolution was strictly in the hands of British owned slave ships. During the Revolution, the British ships obviously quit sailing to Massachusetts, and during that war, in 1780, Massachusetts and Pennsylvania banned the further importation of slaves into their states.
After the war ended American vessels began sailing to West Africa in the slave trade, mostly sailing from Newport RI. and New York. Their markets for those slaves were mostly in the Caribbean, South America, and the American South, specifically Charleston. By 1808, it was illegal for any American registered vessel to carry slaves in international commerce (obviously violated) but it was considered an act of piracy.
But the short answer is that the vast majority of slaves in the US came on British ships before the Revolution.
An accurate accounting of history.