If Real Madrid hadn't wanted him, he'd likely have left for less than was offered last year for him. Ok he took advantage, any manager would have, take off th2 15-20mil overspend on him and Rafa's net spend for the transfer window looks significantly different.
Yes he's taken advantage of other managers awful decisions, Sissoko wasn't worth what you sold him for, neither was Crouch, Alonso or a bunch others. The ONLY area I give him credit for is getting decent cash on players when he sells, but thats mostly down to other managers being stupid, than Benitez being somehow brilliant.