There are different marketplaces such as Amazon, Walmart. eBay, Etsy, Bonanza, Mercari etc. All these are online but cater to slightly different customer base and niches. Though it may be easy to list your products in many or all of them - it is important to know which marketplace is the right fit for your product. Example: Etsy - is mainly for handmade, vintage or craft related products whereas Amazon is for almost anything.