Allow me to offer a correction or two. Slavery was abolished in NY in 1827. Earlier (I think 1799) no new slaves could be brought in. Also, NY had FAR fewer slaves than in the south, where they were mainly fieldworkers, whereas in NY they were mainly house slaves. The census from 1790:
NY had 21,00 slaves and 5 colonies had more than that. See here for source
The Yankees sold their slaves South to the plantations and had no problem with buying cotton, tobacco, and other agricultural products produced with this system. While slavery may have been outlawed up north, many northern cities barred blacks from living, voting, or moving about freely.