Dortmund's performance is not that bad. Im not seeing if selling the players has been preventing dortmund from moving forward. In fact, dortmund can still play better from one week to another week. Im thinking that if what you are saying was not true.
Dortmund has not yet faced hard fixtures and the club is still achieving a very good result as still being unbeaten till this time. Dortmund sees it as the possible business to be running on.
This is also making the club forced to sell its important players. Dortmund needs also to keep its financial to be stable. Using the income from selling merchandise or playing in bundesliga was not enough to make its balance sheets become stable.
This is also the reason why dortmund keeps try to sell its important players. I shall give applause to the scouting team of this club. It makes dortmund was able producing a lot of talented players.
Yes, I also have the same assumption as you, Dortmund's performance at the beginning of the season did not look too promising, they were still below the top five in the standings, but as time went by it could not be predicted that it turned out that they could now crawl up to enter the top five and now their position is in fourth place in the standings, honestly I think this is quite consistent, last season they were the only team that gave strong resistance to Bayern Munich, always chasing in the race for the top of the standings but well as we know they were not lucky enough so that the trophy was again won by Tuchel's squad. For the problem of the business of selling players that they always go through, honestly I will not fully say that it does not interfere, obviously the entry and exit of players is quite a heavy task and of course they need time to start adapting, and I think that adaptation is a tough task for Dortmund, that's why sometimes they are difficult to appear consistent, but well it's not always like that and now Dortmund have proven that they are able to rise to join the title race as usual.
Yes, maybe the business activities that they always do are one of their goals to keep their finances stable and also of course with a little profit, that's possible.