Dortmund have not won the Bundesliga trophy for several seasons, with their habit of selling players it is impossible for them to be able to compete with Munich and several other big Bundesliga teams, even this season alone, Leverkusen's performance is much better than Dortmund, many Dortmund players are being targeted by the team Others, they are good at seeing the potential of young players and developing them to sell at high prices in the transfer market.
If Dortmund have not been consistently selling off most of their players, I'm certain they would have been able to stay ahead of Bayern Munich much enough to be able to able to win the league titles especially last season when they were close to winning and later lost form and Bayern Munich as usual won again.
Dortmund have seem to look like they are more interested in just selling off their players and making more money which seem like it's more profitable to them than winning trophies, so they rather train their players sell them and make more money then just waiting to win the trophy, I think some times they realize good money enough for the players sales which makes them border little about winning trophies else they have got the ability to win the league title several times but have failed to. The business of training and selling player seems to be what they are most interested in.