Spree Commerce VS spryker-core

A good system to study is Spree https://github.com/spree/spree. It has an inventory and billing system and supports different adjustments and sale configurations. If you don't want to use it wholesale it's a solid Rails application that does most of what you want so you can use it as solid inspiration.

Spree Commerce: https://spreecommerce.org/

Maybe Spree?

I'd say Solidus and Spree are you best options rather than trying to roll out you own ecommerce solution.

When I was working on that company, that mobile ordering app, Counterless, I looked a lot at codebases like Spree, the Spree Commerce codebase. And that's how I learned a lot about how do you model an order with line items and things like that? I saw some amazing things that helped me develop an understanding and save a ton of time by looking at oh, they have a state machine. What are the ways that an order transitions through all these different states as it gets processed and charged? And how do you track all of those things?

If you want to learn why your own marketplace may be the right direction for your company, join us for this MFG eCommerce Success show to hear Alex Graf Co-Founder & Co-CEO, Spryker System GmbH talk about the benefits they have seen when manufacturers create their own marketplaces where third parties can sell alongside their products to enrich offering and stimulate growth for their companies.

Spryker is an example of OSS which is not free.