As an entrepreneur, you have to charge a realistic price for your services or goods. You charge VAT, your hourly rate, send your clients invoices, etc. Example: you are a photographer. You take pictures for someone you know, and in return you charge 'a friendly rate' or receive a swap (like tickets to a concert). This is seen as a symbolical fee. In this case, you do not run a business, and you do not have to register with KVK.