A few points I'm skeptical of: simply connect to the seller’s infra instead of locking them in. One of the value-adds of Amazon is there's a whole bunch of tech sellers don't need to know how to write, run, and maintain. You'd need to understand how big the niche "technically literate to run their own e-commerce business, but selling on Amazon for other reasons" is, then how your operation would answer those "other r…
You can always do what bookshop.org did and appeal to social justice and hipster oppression. Talk about supporting "small business" and pretend that everyone's not just dropshipping from asia.
That said, it's still not nearly as good for indie bookstores as just buying directly from the indie bookstore, but it's pretty far from your characterization.