Dortmund president has give the statement regarding why they had a habit always be sell their important players to other clubs this because Dortmund is not the club with strong financial such as Bayern Munich and with selling players to other clubs it can managed the balanced of financial club so that's why although Dortmund has received plenty of critism about their habit then they will always be doing that
Yes, this is a reasonable reason. Even for now, their current transfer record is still in a loss position, with a not insignificant amount, €-22.90m.
Source: Borussia Dortmund if they didn’t sell their stars
Here is their squad list if they do not sell any of their top players so far. However, does not selling them mean Dortmund can be much more advanced? Not necessarily, because once again, Dortmund's finances are not that good to keep those top players, and that is the reason why they often sell their top players, for the sake of their financial balance.