Node.js is event-driven and asynchronous in nature. That's why it's a great choice for I/O intensive highly scalable (realtime) apps. Although we don't see a lot of e-commerce apps written in Node.js, it certainly can be used to write such apps. Some new open source platforms like Reaction Commerce are using it and I don't see any reason why Node.js shouldn't be used in e-commerce solutions. I also don't think concurrent users and cpu are issues.
Big companies like PayPal, LinkedIn, Netflix, Medium, Walmart etc are already using Node.js at scale. Here you can find a list of companies and projects that are using Node.js.
If you want to learn Node.js by creating something meaningful, I would recommend building a basic API driven app. You can also try writing a simple chat app that utilises collaborative editing and other realtime features.