It isn't their joy that they keep selling star players, Dortmund doesn't have any other reliable sources of income than trading with players.
They have no other option for selling the star players as other clubs give them much more money that cant Dortmund affort.
Dortmund has also some nice sponsors and they allways getting some extra when they playing in the Champions League.
Buit yes the most Money and income are for selling players , its sad but they have to doing it.