Its highly advisable to define your Terms and Conditions / GDPR / Policies as soon as you setup your website. If you are trading with customers from anywhere then your ways of working may differ from what they are used to so its better to define those in your T&C's. eg refunds policy - You will always need to have your Cookie and Privacy Policy available on the website and your web designer can help you with that. Your GDPR Policy should outline how you protect and process your customer data as once you start selling you may collect alot of important customer data very quickly.