You realize slavery was outlawed in MA in the late 1700’s, right. Might have been a financial hub, but the slaves were freed pretty early in your timeline.
Yeah slavery was grandfathered out fairly early in some of those states.....but that sure as hell did not stop them from slave trading. Everybody knew it too. They were all too willing to keep pocketing enormous amounts of blood money well well into the mid 19th century.
They supplied the ships and the financing and the crews and everything else for those ships to sail to Africa, pick up slaves, then sell them off mostly in the Caribbean and Brazil.