Atlético are probably willing sellers to be honest.
Aguero's stock is at an all time high, huge transfer fee's are exchanging hands all across europe, Aguero takes up a huge chunk of their wage bill, he wants out (allegedly).
With the money they'd recieve from his transfer they could easily buy 2-3 top class players and pay them all with the money they'd save on his wages.
With the money they'll be getting in from Aguero, Forlan and possibly De Gea they could completely revamp their squad.