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.
But, even though it is clear that Dortmund only wants to do this, but Dortmund has loyal supporters and/or fans. I think that's a great thing, because normally fans will ask for the club to focus on winning titles and not just focus on making money from player sales. I don't know how long this practice will continue to be carried out by Dortmund because of course, if Dortmund continues to do this then Bayern Munich's dominance in the Bundesliga will certainly be difficult to stop.