Hello everyone, I'm new to Solidus and I was wondering if there's a solution for marketplace (multiple-vendor) already. This page seems to show how that is applicable but there's nothing in the documentation or the demo that points to how it can be achieved. This gem has not been worked on since four years ago.

Hello there! As Nebulab, implemented https://github.com/solidusio-contrib/solidus_paypal_marketplace a couple of years ago. We are not actively maintaining it anymore because we are no more using this in any project, but it should be a good starting point. The extension uses some built-in PayPal functionalities built specifically for marketplaces, like payouts and merchant signup.