The question was, who have Chelsea sold, and the answer was Mata, regardless of the situation, they still sold him. That was the context of the question.
Regarding your statement about it being a good bit of business as history has shown, surely history and his stats show that he's one of the top midfielders in the league:
http://metro.co.uk/2015/03/24/missi...ier-leagues-most-prolific-midfielder-5117861/
So, yeah, Chelsea have sold a top player. Surely most players go when the likes of Madrid come knocking? Certainly do at Man Utd