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.
Borussia Dortmund manager Edin Terzi is in ideal positions to achieve his objectives, but the board keeps releasing key players, jeopardizing the club's chances of winning the Bundesliga. If one can recurred the club's past and present deals, they sold out talented players who, if present in the club at the time, would most likely have a different story. Borrusia Dortmund are more concerned with trophies; they need to start winning titles because it's becoming clear that they produce their funds by transferring their talented and promising players to outstanding teams, which is a huge loss on their part.