Northern colonies that had sanctioned slavery included Massachusetts, New Hampshire, Delaware, Connecticut, New York, New Jersey, Pennsylvania and Rhode Island. 

Rhode Island, was among the most active Northern colonies in importing slaves.

In 1700, 42% of New York households had slaves. Seventy years later, there were twice as many slaves in New York as in Georgia. 

The north was also the first to discontinue slavery, not because of moral conviction, but because it became unprofitable due to cheap immigrant labor and industrialization. (Why invest in a slave which required maintenance and care when you could hire and immigrant, work him long hours and if he got sick let him go and hire someone else.) However, the north continued to profit from slavery. First, by selling their unwanted slaves to the south, rather than free them and by purchasing cotton, produced by slaves for northern textile mills."

No sooner did they get the money in their pockets from slaves sold to the South and then turned around and wanted Southerners to free them...how convenient.