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.
Actually, Dortmund is a great team but indeed, Dortmund always fails to win trophies even though they always have the chance. After all, last season Dortmund also played in the Champions League final, while in reality Dortmund experienced difficult competition last season in the Bundesliga standings. Also, even though Dortmund always gets good profits because it always manages to produce great players, but Dortmund also doesn't become bad even though it has sold great players in its squad.
With results like that, at least so far the team still looks fine and perhaps fans will still provide great support for Dortmund even though they didn't get a trophy. Well, I think a team like Dortmund is very rare because if another team sells its key players then the team's performance will decline and get worse, but Dortmund not experienced that, which means that Dortmund is a great team.