There's a number of things misunderstood about fish shops:
People will buy non-aquatic plants, even when you tell them they're non aquatic. If the shop doesn't sell Dracena, they'll go 10 minutes up the road to Pets At Home to get it.
Tanks are often left bare for cleanliness and reduction of aggression. Some sand on the bottom is always nice though, so the fish doesn't feel like they're above deep water.
Dead fish are a fact of life for even the best fish shops. With such large stock, there's bound to be a couppe of weak ones who succumb to the natural selection process. Consistently dead species should be taken off sale until they're shown to be death free for a period of time. Dead fish in the tank for a long period of time is unacceptable.
Large, shared filtration systems are necessary for a stable system. Having each tank on individual filtration is risky when stocks increase suddenly eg. fish delivery day, as ammonia can arise, causing issues. If a shop is kept properly, the systems are regularly dosed with a number of pregentatives, and run with massive UVs to keep disease and parasites from spreading. There'll also be large daily water changes happening. There's a lot going on in those sumps than people see. Selling fish from an infected tank, however, is completely unacceptable.
A bad fish shop, to me, is run by someone who will lie to you to put money in the till. The customer should always be fully informed, even if it isn't profitable or what they're wanting to hear. It's important that people get the correct information in the first place, preventing disappointment and frustration that only puts people off continuing the hobby.