Same thing happened to me but the adopt I sold them was 25$
I know their characters are frozen so I doubt they can do anything anymore but do not sell to them!!
I sold them a character of mine for $15 and about a week later I got a chargeback from them despite giving them the character. Today I learned I lost the dispute and they got back their $15 and paypal took an extra $20 to cover chargeback fees.
DO NOT SELL TO THE USER SHARKMILK IF YOU CAN AT ALL COSTS