I mean it recent times. Apart from the Adebayor and Assou-Ekotto situations where they literally had to pay them to leave they've sold Fazio, Chiriches, NJie, Soldado, Paulinho, Capoue, Dempsey and Holtby for a loss. The same will probably happen for Janssen and Sissoko.Sp**s made a ~70M profit on Bale alone, don't think they're constantly making losses on their players.
It helps that those clubs have had new managers come in as well, with no attachment to the players they inherited. Potch sold most of AVB's dross, Guardiola got rid of Pellegrini's etc.The point @Toast is making is that regardless of losses, these are revenue streams for clubs. Revenue streams where we are pathetically behind our rivals.