Payment processor, or marketplace?

Payment processing is a lot of work with compliance. That’s one reason that it’s centralized.

From the Awesome Self-hosted list here check under E-Commerce

https://github.com/awesome-selfhosted/awesome-selfhosted?tab=readme-ov-file

https://alternativeto.net/software/shopify/?license=opensource&platform=self-hosted

I don’t have any experience here personally, but alternativeto.net always has good starting points.