Maybe their fans are good (and that's a nice touch) but odd willingness to sell their best players to Bayern isn't exactly making the Bundesliga more competitive. That's the last club they should be selling players to. Infact when they sign players they should go so far as to make them sign a contract clause that they can't seek a move to Bayern. I certainly don't see City selling to United anytime soon (or vice versa) and it should be the same there. Dortmund fans should be extremely unhappy about it. Of course finances are weaker at Dortmund but it still shouldn't excuse it.
Can't remember exactly when but Klopp was quoted as saying Bundesliga was most competitive league in Europe. Well its changed since then and selling best players to your arch rivals isn't going to help.
No idea why their fans tolerate Lewandowski's presence. They have less tolerance for high ticket prices in Germany so why tolerate someone like that until summer and he had the nerve to say it would be unfair for them to boo him.