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.
They have to sell players that do not want to stay, and they are paid so well for this players by the clubs that buy them, so Dortmund can also invest that money in other players, they didn't lose their players for free. Bayern Munich are so strong in the German league and Dortmund wouldn't have been able to stay ahead of them even if they haven't sold any player in the last 3 years or so. I want some other team thar's not Bayern Munich to win the league this season, Leverkusen are doing very well and they are top of the German league, but i still think that Bayern will come back to win the title.