It's always responsibility of the buyer to quarantine new fishes. There are zero marine fish shops here that hold fish for a full month of quarantine (yes that is how long real quarantine takes) before selling, fish will end up being too expensive for the market here to accept. There also can be the best copper systems, uv systems put in place but if the fish already arrive with diseases from the supplier there is nothing much the importer can do. If you want to be safe, pay for the fish fully and have the shop hold the fish for a week. If there are any signs of whitespot/velvet it will show up by then and it's a measure that could end up saving your entire collection.