Actually even after the shipping of slaves from Africa officially ended, Yankee traders still profited legally from slavery by exporting slaves from the more northern southern states to the more southern states when the economics started failing on the tobacco plantations. And then there was the illegal slave trade:
Even after slavery was outlawed in the North, ships out of New England continued to carry thousands of Africans to the American South. Some 156,000 slaves were brought to the United States in the period 1801-08, almost all of them on ships that sailed from New England ports that had recently outlawed slavery. Rhode Island slavers alone imported an average of 6,400 Africans annually into the U.S. in the years 1805 and 1806. The financial base of New England's antebellum manufacturing boom was money it had made in shipping. And that shipping money was largely acquired directly or indirectly from slavery, whether by importing Africans to the Americas, transporting slave-grown cotton to England, or hauling Pennsylvania wheat and Rhode Island rum to the slave-labor colonies of the Caribbean.
Northerners profited from slavery in many ways, right up to the eve of the Civil War. The decline of slavery in the upper South is well documented, as is the sale of slaves from Virginia and Maryland to the cotton plantations of the Deep South. But someone had to get them there, and the U.S. coastal trade was firmly in Northern hands. William Lloyd Garrison made his first mark as an anti-slavery man by printing attacks on New England merchants who shipped slaves from Baltimore to New Orleans.
Long after the U.S. slave trade officially ended, the more extensive movement of Africans to Brazil and Cuba continued. The U.S. Navy never was assiduous in hunting down slave traders. The much larger British Navy was more aggressive, and it attempted a blockade of the slave coast of Africa, but the U.S. was one of the few nations that did not permit British patrols to search its vessels, so slave traders continuing to bring human cargo to Brazil and Cuba generally did so under the U.S. flag. They also did so in ships built for the purpose by Northern shipyards, in ventures financed by Northern manufacturers.
In a notorious case, the famous schooner-yacht Wanderer, pride of the New York Yacht Club, put in to Port Jefferson Harbor in April 1858 to be fitted out for the slave trade. Everyone looked the other way -- which suggests this kind of thing was not unusual -- except the surveyor of the port, who reported his suspicions to the federal officials. The ship was seized and towed to New York, but her captain talked (and possibly bought) his way out and was allowed to sail for Charleston, S.C.
Fitting out was completed there, the Wanderer was cleared by Customs, and she sailed to Africa where she took aboard some 600 blacks. On Nov. 28, 1858, she reached Jekyll Island, Georgia, where she illegally unloaded the 465 survivors of what is generally called the last shipment of slaves to arrive in the United States.