Copper is pretty reactive to alot of stuff, so my guess is whatever this coin was cleaned with tarnished it. Since it is listed as cleaned, I would say someone used a chemical the coin did not like.
The coin has been dipped in one of any number of substances and has started to retone. The darker areas are where the toning turned into corrosion and ate into the surface. 98.265% sure.
My posts just about never come out right the first time. Sometimes I think my fingers are faster than my brain. lol
p.s. FWIW, that's an 1842 small date -- a $1000 coin in 63bn. $50 for a cleaned example really isn't a bad price, IMO.