Borussia Dortmund has built up a good squad depth that's capable of challenging Bayern Munich for the German Bundesliga title every season, but since Borussia Dortmund is in the business of selling their players for money, their performance has slowly declined, leaving them to be less of a threat to Bayern Munich in the race to win the German Bundesliga title. If Borussia Dortmund don't sell Guirassy this season, just know that it's not yet time for Borussia Dortmund to sell him.
I disagree, they are not building the team they need. They keep finding good players but then they end up selling those players, and not use the money well enough. Most years they make more profit than loss, and that is not a good way to grow for a team. They should be making their profit from merch sales, ticket sales, tv rights, sponsors and so forth, not from player sales. Look at all league winners, they end up spending money on the players and do a lot better because of it.
So if you do sell 100+ million worth players, you should buy 100+ million worth players, be at a loss from the transfers and try to grow. They are not good enough, and they need to try to be a lot better than what they have right now.