Wordpress is a well-established Content Management System (CMS). It was originally designed as a blogging system, and can still successfully be used for that purpose, as it produces easy to use RSS feeds and an excellent commenting system. However, it has grown to include static pages and menu systems, so it is now often used as a fully fledged CMS. It is open source.
Open Source does not imply that it cannot be sold. Many developers have taken the basics from Wordpress, then tweaked them for their own purposes and sold the end result. The customer is buying the developer's tweaks, and his time, and an honest developer will make that known.