The word sale means the act of selling goods or services. The tag "for sale" is used to identify that something is available to be sold.
The internet began as a method to for scientists and scholars to share ideas. More recently the concept has expanded to also sell goods and services, and share information about those goods and services — both online and at physical "brick and mortar" locations. The sharing of discount items and services "for sale" has blossomed over the past few years. Websites and blogs discussing real estate, clothing, toys, odd stuff, and many other goods "for sale" have also flourished.