Petsmart and Petco do NOT sell dogs or cats. The people you see out front on Saturdays and/or Sundays with dogs are from licensed animal rescues and humane societies. They also do not "sell" animals. There are more strict rules governing the adoption of animals than there are the purchase of animals.
You cannot adopt an animal from a rescue and turn around and re-sell it when you can't care for it any longer or don't want it. That is a breach in contract and a punishable offense.
Rescues do not "sell" animals. They take in animals others don't want/can't afford, spend $$$ on its treatment, getting it healthy, training, etc. and then adopt it to a loving home for a fraction of the $ they put into it in the first place.
The animals you see outside Petsmart and Petco are not returned to plastic cages in the store at the end of each day and brought out like circus animals "for sale." They go back to the shelter or foster home from whence they came.
Sadly, there is an animal rescue for almost every single "pet" (and sometimes not pet) species of animal in the world. That should tell you something.