Codecanyon is a market place where you can sell and purchase WordPress themes and plugins.
If you are a WordPress developer and want to make some more money, codecanyon is the best option for you.
Read This Before You Start Selling on Codecanyon
Make a research on what is trending, write the script for WordPress and add a listing to the codecanyon.
Market your product/script so more people can know about it and buy from you.