In the case of Dortmund, I can say that the club's owner and coach may know what they're doing because every year during the transfer window, they sell every young star player. I can also say that once players have developed into very talented with football, they are now sold them, because the club's owner takes the business of football very seriously. People may be doubtful of their performances when they sell popular players, but they have come up with an strange performances every year to compete with Bayern Munich for the championship. No one knows when the behavior in that club will end.
The only issue with selling young stars for a lot of money is that they do not have an endless pit of superstar young players. Looking at their club right now, Schlotterbeck is 23 and seems like worth 40 million, not sure if he is but that is the biggest they have so far, Adeyemi maybe at 35, Moukoko is 18 years old and 30.
These are the only three young players end they are 40-35-30 million worth, if they sell them all at the same time they only make as much as what they made from Jude, and maybe they can get more than what they are worth at transfermarkt, which is not unheard of and could happen, but I do not think that they would get 100 for any of them, it is just not going to happen. You are not going to keep selling players like that.