I think it may be possible to build the marketplace in a purely FOSS-y way, but it would be illegal to operate it with the wrong configuration values. I'm thinking in particular about the accounting functions, such as earmarking x% of each sale for royalties, and ensuring they go to/from the correct bank accounts.

In other words, one could plausibly release the code as FOSS, but the interface would depend on a set of corporate entities that are configured a particular way, so it would be of limited value to the median person.

It would definitely lower the barrier to entry for people to fork the business, though, which is probably a good thing for the median person.

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I'd also be down to contribute as an engineer, if such a project already exists with momentum or if somebody wants to start it!