The first step is to identify talented players before it becomes obvious. That's the part Dortmund do so well. Pulisic might be overpriced but he's also a good player, and potentially a very good one.
We don't sell well, by and large, because we're trying to sell players that have proven to be unsuccessful (talent/coaching issue) in the very league where Dortmund take clubs to the cleaners.
Other Premier League clubs would pay big money if we put Bellerin (injury aside) or Guendouzi on the market. Because they are young, hugely talented and have proven they can play at the top level. We wouldn't get bumped in those cases.
You can't sell wastrels, or good prospects you've coached to play like wastrels, for big bucks. Even Premier League clubs aren't that stupid.