A retailer is a company that buys products from a manufacturer or wholesaler and sells them to end users or customers. In a sense, a retailer is an intermediary or middleman that customers use to get products from the manufacturers. They sell goods directly to consumers through various distribution channels with the goal of earning a profit. This merchant can be a physical building or online. In general, retailers don't manufacture the goods they sell. There are some exceptions to that rule, of course, but usually, the retailer is just the final link in a supply chain that gets a product to a customer.
Most vendors you will work with will require you to have an EIN before doing business with you. Having an EIN means that you'll be responsible for a variety of taxes, so know what your local and state taxes are, and become familiar with what you'll be expected to pay the federal government. Check with your local Chamber of Commerce if you're unsure how to proceed.