They've made some cash selling hordes of mediocre players who were occupying their reserves or underage squads. But ultimately it's possible because they haven't had hardly any talent come up internally, have bought near finished products, and have never needed to worry about wages to keep them happy. They've generally been able to hold onto aging assets without worry that their sell on value is declining like Bravo, Otamendi, Mangala, Delph, etc. Their fringe players at best and would have been sold by most other clubs to make money. City can just keep them around without worry.