Ok, LNbits is going in the right direction. It's a specialized nostr client that's basically a shop/marketplace. A lot of the functionality seems to be running in DMs. Nice.
the link to the video:
https://piped.kavin.rocks/watch?v=lqV0XkOlcCs&t=2998s
What I had in mind was something that would just extend the twitter-like clients to be able to search for notes that contain "a marketplace" content. While the buying/selling would look like two people discussing the details in DMs. Somewhat everything-app like and simple.