I have purchased most of my fish from Petsmart. The ones I did get from there are either still alive or died from causes other than disease. By contrast, I did recently buy 4 fancy guppies at a specialty fish store near my house. All died within one week.
My suggestion for petstmart fish buying. Find out when the stock comes in. The fish come in to my local PetSmart on Thursday mornings so if you do stop in on Thursday night you will see the tanks packed to the gills and alot of dead fish. Just remember alot of fish do not survive the transport process to the store due to stress.
How would you like it if you were born in an overcrowded tank, raised in an overcrowded tank, then one day boxed, bagged, and shipped. Not too happy I will assume.
So what do I do when its time to get new fish, wait till friday night. If they are still alive 24 hours after being placed in the display tanks and they do not look sick, chances are the fish is pretty healthy.