I wouldn't call them backyard breeders, and the problem isn't unique to backyard breeders. The problem is real ANY time you select for ANYTHING other than health/hardiness. The best dog breeder in the world is still causing the exact same problems. Maybe they are okay for the generations they have produced so far, but what they are doing is causing harm for the overall health of the breed. The same is true of goldfish. The high end ones are super sensitive. I have heard the same thing of guppies, bettas, on and on. When you select for color, shape, size, etc. you are breeding in spite of health/hardiness instead of for it.
Which shop?
How fast they sell depends on a lot of things. Their traffic, how many they have, their pricing, size, color, etc. I would ask how many they have, how long they have had them, what all they are eating, etc. You can also ask them how fast they sell. They may not be completely honest and just try to get you in, but it won't hurt to ask.
You presuppose that it's not possible to select for more than one trait at a time. 100% incorrect. You presuppose that color, conformation whatever are automatically at odds with hardiness, suitability for captivity, whatever. 100% incorrect as well. Colorful Discus are much easier to breed than their wild progenitors, so obviously while selecting for color breeders were selecting for ease of breeding as well.