E-commerce is the name used to describe the features of a web site which enable people to sell things.
E-commerce solutions vary enormously. At the simplest extreme, they might entail an author signing up for Amazons associates program, and providing links to the relevant places on Amazon to enable people to buy their books.
Other simple solutions involve people selling things using Paypal to collect the money: Paypals Buy now buttons providing a good way for the visitor to order something quickly and easily.
More sophisticated solutions involve electronic shopping carts, so that people can order a variety of goods, and interfaces with online credit card payment services so that those goods can be paid for. If you are interested in an E-commerce dimension to your site, do get in touch. Do bear in mind that it can make sense to start with a modest e-commerce component to a site and to let that grow as the site, and the business, develop.