I've been looking for such open source software and everything pointed me to opencart. Despite my general dislike for php I thought, it can't be that bad if its so popular right? Wrong! Some core functionality (multilanguage support) is so buggy I'd spend most time fixing those bugs than writing my extensions.
Next was Saelor. I love python and I really liked the speed of their front end, but it seems about a year ago that front end (react-storefront) went through a rewrite and there is absolutely minimal documentation. And just reading the code it seems to be beta software at the moment still not ready for production.
I wanted this to be simple, but I might have to look into bigger products like magento, woocommerce, despite the fact this shop will probably not exceed 10 products... If anyone has any advice (other than write your own) I'm very interested in hearing it.
Oh, BTW, why open source? I want to be able to extend the functionality and fix bugs myself.