Gumroad
Sell stuff and see what sticks
...The UI offers product pages, carts, and dashboards for creators to manage listings and see performance, while APIs/webhooks enable integrations with external sites and automation. The codebase demonstrates practical patterns for multi-tenant architecture, permissions, testing, and asynchronous processing at scale. For engineers, it’s both a working marketplace and a study in how to balance flexibility for creators with reliability in payments and delivery.