The sign is intended for tourists who think that the animals at the store are for sale as pets.
Different cultures eat differently. San Francisco’s Chinatown attracts a wide variety of visitors, including tourists from many cultures. They may not understand how the Chinese eat. This is the reason the store owner put up the sign, to prevent non-Chinese from trying to buy his food as pets.
Farther down the street was Ping Yuen Fish Market. The front window displayed a tank crowded with doomed fish and turtles struggling to gain footing on the slimy green-tiled sides. A hand-written sign informed tourists, "Within this store, is all for food, not for pet."
Waverly is aware that the fish and turtles are being sold to make dinner. Turtles are commonly used for soup, and of course the fish could be eaten any number of ways. The sign is an example of the cultural divide between the Chinese Americans and natives. Some people might think that killing turtles is cruel, for example, because they do not understand that they are often eaten in many cultures.
On less crowded market days, we would inspect the crates of live frogs and crabs which we were warned not to poke, boxes of dried cuttlefish, and row upon row of iced prawns, squid, and slippery fish.
This colorful bit of insight into Waverly’s neighborhood provides good background for the setting. The store is a popular one, and we know that the reason the fish and turtles are still alive is that people want them to be fresh. They are fresh because the butchers wait to kill them until just before they are eaten. They do not kill them until they are purchased.
Waverly’s mother uses the crabs as a lesson, telling her that a little girl who ran into traffic was squashed flat like a crab. This makes her nervous around the seafood. It is another example of Waverly’s personality, and her mother’s.