It is common for WordPress plugin developers to sell a free version of their product (albeit limited) in order to get people to use and test their product. Under this freemium business model, many users are encouraged to upgrade to the more feature rich premium version.
Developers who sell multiple WordPress plugins normally create an online shop which lists all of their free and premium WordPress plugins. This page lists many of these WordPress plugin shops.
Be sure to check out my list of WordPress plugin marketplaces too.
Barn2 Media is a WordPress web agency based in the UK that have a large collection of WordPress plugins.
Their best selling plugin is WooCommerce Product Table. The plugin can be used to list products in a searchable table with filters. Other plugins include Password Protected Categories and Better Recent Comments.
BuddyBoss has a large collection of premium WordPress plugins and themes for the popular WordPress community plugin BuddyPress.
Plugins include an inbox system, a Facebook inspired community wall, and a member types solution.
JoomUnited are a company that sell premium Joomla and WordPress plugins. They boast a collection of solutions such as a table manager, file download, and a stylish latest posts plugin.
Limited versions of their plugins are available to help you try their plugins free of charge.
WPMU Dev boast a huge collection of premium WordPress plugins and themes. Everything is included in their $49 per month membership fee.
In addition to their products, they also publish articles and tutorials regularly on their blog. A WordPress academy is also provided to members that helps you improve your knowledge of WordPress.