This is really cool, exactly the kind of thing "only Nostr" can do.

My first glance question is: Isn't this an ideal application for multisignature Bitcoin? This is basically a 2-of-3 transaction between buyer, seller and arbiter.

Also is "Kind 3400" already in the protocol or something you came up with?

Are / should transactions be linked to these Nostr events directly in some way?