We ran an e-commerce mobile app. There were no real tools for this; things like Shopify were designed for web.
So we built a whole shopping cart and inventory management system from scratch. It was good enough that people were willing to pay for it. But we were only 98% confident in it so we kept it internal.
We also didn't have the funds to develop or keep testing it. If we shut down the company, I might open source it.