A real sacrifice on their part, no doubt. Rather like England abolishing slavery domestically.
Well, no, Massachusetts would be more like England, since Boston rivalled Charleston as a slave port at one point in the colonial era, but they didn't keep many there, just profited from the sale of them, sent them south and bought the goods produced by the labor of the slaves they'd sold.
Yes, 'twas very brave. Pure as the driven snow, too.
Historian Richard Hofstadter, in his posthumous America at 1750, informs us that Rhode Island was the slaver home-port of choice.
American bottoms carried about 1/12th of the slave trade in colonial America, the great bulk of the rest of the slavers being British. Of the American proportion, far and away the majority were Rhode Islanders.
American slave ships were generally much smaller than their British counterparts, being about a third the tonnage. This was a deliberate practice based on familiarity with the mathematical principle of "gambler's ruin", since each ship would otherwise represent too great a wager on every voyage.