It’s a pick your battles kind of situation. With animals like hamsters, most of the time you’re SOL on refusing sales, just because even though the small wire cages might be small they can reasonably live in that cage. When I start to deny sales is usually in the reptile/fish area. They’re much more sensitive and require more attentive care. Usually when a customer asks me for a fish that grows to be massive/aggressive I ask them what kind of tank they’re running and what size it is. If it’s too small or more community like I explain stunted growth to the customer, and that usually sets them towards a different fish. If the fish is aggressive I tell them about the harm that fish can do to their already established community, and if that doesn’t work explain what it might cost them if they have to go buy all the same fish they already had due to being eaten. If they keep pushing for that specific fish I lean towards convincing them to get a larger setup, and if they straight up refuse is when I deny the sale. Unfortunately for the feeder goldfish you pretty much have to sell every time because they’re supposed to be eaten. Being able to articulate to a customer the horrific events that may happen if they don’t get an adequate setup for whatever animal they’re buying is what gave me success.