Is the 'no photography' sign valid in the supermarket?

You can take photos in stores. It is legal even if there is a sign saying it is not allowed. According to Ukrainian legislation, the store is a public place where anyone can enter, and where neither photography nor videography is prohibited. Therefore, it is possible to photograph price tags, goods and shop windows without hiding from the security guard. No one can prohibit filming in a store: neither a cashier, nor a consultant, nor a security guard, nor even the owner of the establishment. If they say that photography is prohibited by law, they are simply misleading you. There is no such law in Ukraine. The buyer can be prohibited from taking photos and videos only if the flash can damage the goods, or it violates public order, disturbing other buyers or sellers.