Others develop the products. In most cases, you don't even have to buy the product to sell it to others. Most affiliate programs don't charge you anything to join. You can easily create an affiliate program to sell your products also. We develop special expertise in subjects where we have a strong personal interest.
Information is the engine that motivates the Internet.
Developing your passion is the key to success. Start with your passion and what you already know, and then develop your own product line. Information products are the most profitable products in the world. People who want to know what you already know will purchase your information product. You don't have to be a writer or a world-class athlete.