Gumroad
Sell stuff and see what sticks
...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. For engineers, it’s both a working marketplace and a study in how to balance flexibility for creators with reliability in payments and delivery.