Best auth for B2BC use case?

I’m trying to understand what the best auth solution would be for my e-commerce app (React).

I have two types of users: sellers and buyers. Buyers can buy on their own or through a phone agent (“seller”). It’s a single-product purchase process, and the app fundamentally requires no auth or buyer account features (i.e. attribution > security). The flow is identical for buyers and sellers.

However, for seller-assisted phone sales I need to know which seller assisted which buyer. So, my attribution solution is to have only sellers authenticate.

Sellers shouldn’t be able to sign up on their own - this needs to be a simple (email and/or phone) allowlist process: a seller reaches out and an admin enables their access in some dashboard. (Changes to seller credentials would also be manual / by admin).

I’ve ruled out Auth0 & Okta already due to their complexity relative to my minimal use case. I’ve also been thinking about making it magic link only, but am aware how annoying these can be, esp if the email doesn’t arrive right away.

I’m also feeling that given how manual the signup has to be, username & password processes may also be unnecessary.

A social MFA would be nice and it’s ok for sellers to use their personal gmail, but many sellers won’t be gmail users and may use yahoo or (gasp) Hotmail or worse.

This brings me back at magic link (and/or phone auth, for example with firebase), but it’s not clear to me which providers support the admin-driven signup out of the box. My app is complex enough so I’m looking for something that’s rather plug and play…

Any suggestions for my (somewhat odd) use case?