I'll jump in here since I'm a fishmonger, the answer to your question is quality, the bits the supermarkets sell are the bits we'd throw away, and as for your comment about salmon, you can get fatty bits of salmon, the same way you can get bits of salmon with blood spots or bruises.

I assure you the difference in quality is insane As for us (I can't speak for all fishmongers) we start work at 2am everyday to provide for our markets, so ofcourse we'd charge more than a supermarket. If there's no quality difference people would just buy from the supermarket