In the US you are not required to have a license to sell vitamins and supplements online. However, the FDA has regulations that are applied to dietary supplements that manufacturers must meet. If you want to create your own formulations you will need to get a certificate and have the appropriate insurance. An easier thing to do is approach a manufacturer that will do private labelling for you. These companies are already set up to make the supplements, they simply add (for a fee) your labels to the packaging instead of their own. While this approach can be expensive, you have to remember that these manufacturers are the ones worrying about licensing and logistics and not you, so that is worth the cost.
Aside from ingredients, wording and labelling are very important factors to consider when selling your supplements. Ingredients obviously have to meet regulations and your products can’t contain banned or illegal substances. But you also cannot make any false claims about what your product can do. The FDA pays careful attention to the claims of health supplements to ensure the general public that the products are safe and effective. You shouldn’t say you can “cure” or “heal” a disease or illness. Your labels must also follow the FDA’s structure guidelines. The labelling is the most crucial part of selling supplements properly within the FDA rules.
Insurance is a key thing to have. You will need to have liability insurance to protect yourself should someone say they got sick or injured from your product.
Using an existing manufacturer with your labelling is probably the easiest way to sell supplements. The ingredients have already been approved, everything has the correct certification and is ready to go. All you need is correct labelling and insurance. Some people suggest creating a corporation for your supplement brand ensuring you are not personally liable in legal cases which is definitely something you should explore if you want to sell vitamins and supplements in the US.