Selling or buying products or services online via applications or website portals are termed as eCommerce. These eCommerce transactions may be business to business (B2B), business to customer (B2C), customer to customer (C2C) or customer to business (C2B).
E-commerce is what helps to sell things on the Internet. If you order something on Amazon, for example, this is e-commerce. On this site https://elogic.co/services/bigcommerce-development/ there is a lot of information about this and about how people improve their sites in order for sales to go well.