I don't know what all this 'selling club' stuff is about.
Every club gets rid and every club brings in, and latics did it well because it was advantageous at the time.
City got rid of De bryne, utd got rid of Ronaldo, liverpool got rid of Suarez...3 very effective players.
We got rid of some to bring in money to compete with clubs that had ten times our income ,it's natural.
Real madrid(biggest income club) sell players because the player wants to go, so Real madrid are a selling club.