The main question is, what is the quality of the water, and what conditions are the fish in? Not just the ones you want to by, but all the fish in the tank. (You still need to quarantine any new fish.) Dirty water, dead fish floating around, grossly over crowded…. these should throw up a red flag. I usually spend a long time watching the fish I intend to buy, how it swims, how it holds its fins, how active is it…etc.
I cannot answer for your store, but I have found in my area that the low-priced fish stores usually do not have quality fish (by that I mean quality standards). I think they buy culls from breeders. There is nothing wrong with the fish and I have bought them myself. In my area I can buy 4-5 inch comets for US$3 and the same size comet at another store for US$7. Am I being ripped-off? Well, the expensive comets have tails almost as long as the body, as they should be. The cheaper comets have tails half as long as the body. They are still beautiful fish. The choice is yours.