Selling products for pets, or selling pets themselves is different and require very different infrastructures for live let safety:
- I suggest keeping this tag "shop=pet" only for shops that really sell live pets (along with attributes "vending:food:pet=yes", "vending:accessories:pet=yes" for animal shelters or toys or similar, "vending:chemistry:pet=yes" for veterinary chemistry products including ani-pest products, "vending:hygiene:pet=yes" for animal hygienic products, "vending:care:pet=yes" if there's some well-being care practiced on animals, and other similar "vending:*" keys, only respecialized for pets rather than humans by default).
- For those that only sell products for pets, it should be "shop=pet_products" (along with the same attributes).
- Those attributes above will also be commonly found on "amenity=veterinary", or for more general stores selling prodcuts for humans (if they also sell pets, they can set "vending:pets=yes", see amenity=vending_machine whose "vending:*" subkeys could take most values currently used for the shop=* key).
The alternative would be to use the current tag, but with "vending:pets=no" (expecting that "shop=pet" is really selling pets, i.e. vending:pets=yes implicitly).
Note also that the term "pet" is a bit too restrictive where it should be just "animal". Not all animals are used as pets at home, they may be purchased for agriculture/gardening, or sport, and living freely outdoor, or used to feed other pets/animals already owned by customers (the animals sold in these shops should be live, otherwise they are just "food" for pets/animals)...
Also "pets" or even "animals" should be replaced by more precise animals if needed (such as "dogs", "pigs", "cats", "birds", "horses", "fishes", "reptilians", "insects"/"bees", "spiders", "worms"...).