Do you think that yankees only purchased slaves in 1800? They did so in 1799, 1798, 1797 and decades further back, and in 1801, 1802, 1803 and up to the war. The slave trade was the cornerstone of the northern economy and the Triangle trade.
Oh, so to get "millions of yankees", you count the same yankees every year, huh? I guess that would put the number of southern slave owners in the billions.