I don’t subscribe to the outdated “customer is always right” mantra, but it’s generally bad practice to insult customers regardless of their behavior. State your requirements for purchasing, and if they don’t want to comply and/or become hostile, politely and professionally provide them with the opportunity to leave. This way you never need to worry about being perceived as an asshole by other customers.
I don’t subscribe to the outdated “customer is always right” mantra, but it’s generally bad practice to insult customers regardless of their behavior. State your requirements for purchasing, and if they don’t want to comply and/or become hostile, politely and professionally provide them with the opportunity to leave. This way you never need to worry about being perceived as an asshole by other customers.