Gumroad
Sell stuff and see what sticks
...The app models products, variants, subscriptions, and checkout flows, then ties them to fulfillment, licensing, and customer communications. It implements typical marketplace concerns—order management, refunds, discounts, taxes, fraud checks, and analytics—through well-defined services and background jobs. 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. ...