I'm currently building an open source penny auction site. Since I like NodeBB and the way its code is structured, I reused some parts (templates.js, socket io, the way middleware, routes and controllers work) and it works pretty well so far.
If anyone would be interested in developing for fun, I'd be happy to share the codebase on GitHub. It's an interesting project so far that I've learnt a ton of stuff from, and there's still a bazillion things left to do.
Some screenshots from the Admin panel (front-end is german, but that'll change once I figured a way to make translation work smoothly):