I think it's naive to believe that informed people 'chastizing' a chain like Petco or Petsmart by not buying animals from them will make a difference.
However, buying an animal from an individual Petco or Petsmart that IS doing things right, and letting them know that you appreciate it, can ensure that they continue to pay attention to it, and do things the right way.
(In other words, you aren't going to impact the big chain by doing nothing but refusing to buy from them--but you will impact individual stores with your support. In most cases, doing something will always make a much bigger impact than doing nothing. Doing nothing is just a lot easier).
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