Ask HN: How can I being a developer help my girlfriend sell art online?

There are mature platforms out there that act as a platform to showcase and sell art out there. Building your own platform might be interesting as a project, but not economically sensible. An existing platform gives your girlfriend the tools to interact with people who like her art, and to experiment without having to wait for you to finish a new feature.

If her art is physical, consider Etsy; if it is digital, look at DeviantArt, Patreon, or one of the many order-to-print shops.

Ebay. The hard part of selling art...or anything...isn't building a website. That's just pretend work that's easier than selling. Good luck.

as shop platform you could use something simple like ebay but you need to get very good discoverability of her works to find buyers - probably good solutions is to use many social sites like Instagram, deviantart..., therefore you can build (or find) tools to automatize posting/managing images+descriptions to lot of places at same time.