Dortmund will never be able to remain silent in the transfer market, where they always cash in on their mainstay players, in fact this has become their tradition. In every season, Dortmund always succeeds in turning other players into star players, so in this case they have managed to balance quite well. But what Dortmund management is doing is certainly not a good thing for fans because fans always fail to see Dortmund become champions.
After signing Serhou Guirassy, they needed to sell a CF as the number of forwards was like 4. They chose to sell Füllkrug after getting a good offer for him. Otherwise maybe Haller might be the one leaving. Because Füllkrug had a really great season so he would still be very useful as a backup striker. Anyways, in the end they are always ready to fill someone's space quickly.
This is why it isn't much possible to see them silent in a transfer window.
This is their main job though, making big profits in player deals. This is like their main source of income while not being able to win a Bundesliga title.