Compared to whom?
Everyone sells, a lot. We're no worse than anyone below us. City and Chelsea normally only sell when they have no use for a player, but it's a myth that we sell more than other teams.
We sell players when they want to leave, or when we have no use for them, or when the offered value is greater than the value of the player to the club. ONLY Rooney was sold for a bad price. Most of the time we make out like thieves...which we should, but don't, invest.
We don't buy, and we don't sell much, and we certainly don't invest, but it's just incorrect to say we sell more than other clubs.