Postman - The Collaboration Platform for API Development
OpenCart - A free shopping cart system. OpenCart is an open source PHP-based online e-commerce solution.s
Django REST framework - Django REST framework is a toolkit for building web APIs.
Sellfy - Sellfy is an easy to use but powerful e-commerce platform where you can sell digital, print on demand or subscription products. Use a Sellfy storefront to create a beautiful online store or embed a Sellfy shopping cart to your own website.
Amazon API Gateway - Create, publish, maintain, monitor, and secure APIs at any scale
Gumroad - An all-in-one solution to sell your work and grow your audience.