As you say

Of course, turning this concept into a fully functioning marketplace would require serious development. There would be challenges involving escrow, dispute resolution, seller verification, shipping, fraud prevention, and the overall user experience.

And I think that the previous attempts haven't recognised this. But now that development is so much easier, the resources to provide a simple and wide-reaching platform are much less than they were. I reckon Claude or similar could turn your idea into a PRD to at least assess what those resources are.

!BBH