Final answer:
Distributors are required to verify refrigerant purity before selling it to ensure compliance with environmental standards and prevent harm to the ozone layer.
Step-by-step explanation:
Distributors must verify refrigerant purity before they can sell refrigerant. This is because the sale and distribution of refrigerants are regulated to ensure compliance with environmental protection standards. Impure refrigerants can cause significant harm to refrigeration systems and the environment, particularly the ozone layer. Therefore, to avoid the illegal venting of refrigerants and to protect the environment, distributors are required to ensure that the refrigerant is of acceptable purity and that it meets certain standards before selling it. It's not about the customer's age, environmental permits directly, or the payment method at this verification stage.