Awesome question.

I like a friendly store which knowledgable staff. I also am looking for clean tanks, good maintenance, not a bunch of dead fish.
I always ask when the fish shipments came in. I don't want to buy fish the first day they were put into the store's tank, because it is too traumatic for them to
acclimate to the store's water and then yours in the same day. I also ask how long those particular fish have been in the store, any illnesses or deaths in the tank, etc. You need a store that you can trust to answer these questions honestly.