Pennywise - Cross-platform application to open any website or media in a floating window
Gumroad - An all-in-one solution to sell your work and grow your audience.
PiP for Mac - Always on top window preview similar to the popular windows only OnTopReplica
Payhip - Sell digital downloads, courses, coaching, and more from one simple platform.
Shopify - Shopify is a powerful ecommerce platform that includes everything you need to create an online store and sell online. Try it free for 14 days.
robora - Robora is a service which aims to help you sell anything, anyway you like.