Would it work to also charge a modest refundable fee (say $1 per month) from sellers via Stripe or someone else who does KYC, and then use that as a base of "this person has been flagged as known" to solve most KYC concerns in this post?

Yes, this is exactly what it does! To confirm, Zoneless does have KYC built in - its a bring your own API key system that plugs right in, and you get 500 free verification checks per month via Didit. There are guides in the docs on how this all works.