It's hard to always get healthy animals when dealing with a middleman. It's like buying a puppy from a petstore, more often than not it'll come home with kennel cough at the least.
At breeders/hatcheries biosecurity is in effect, where as at the store, people come in all the time and want to pet/hold/touch the chicks. Where have they been and do they have chickens and what all was passed around? Or where did the chicks come from? Did the store clean the brooders from the last batch?
That isn't normal mortality regardless of the cause, but the more times chicks change hands, the more likely it is they can become ill.
The safest way to buy is from the egg straight to you, either from a hatchery or from your own incubator or that of a breeder.
Unless the local store is VERY reliable and has a good reputation. But the people who work in the store, are not going to be chicken experts until they've delt with them AND their potential issues often enough to gain experience. Alert the store to the chicks being ill.
