The whole team in Dortmund is based on selling players to other teams. As you already said, they are not a team with strong financial power like Bayern Munich. Dortmund's first priority is not even winning the Bundesliga title but they want to sell players and earn money and that's how everything works in Dortmund.
The interesting thing about Dortmund is they even have more fans to watch games in the stadium which can show they have much more potential than other teams in Germany.
Dortmund is a team that don't like keeping players, they always like selling out their best players after every season, they don't care their fans reaction and they don't care about the team and they equally don't care about winning the Bundesliga.
They always try their best to make effort towards winning the Bundesliga, but they do fail because of the players they always sell, of not they should have won several Bundesliga titles ahead of Bayern Munich because if you rate the players they sold you will know that they should have goos squad if they have kept this players; nevertheless, they are prepared for a fresh season, lets see how the competition will go between them, Bayern Munich, and Leverkusen.