This is 'The Anti-Amazon Alliance' as Ben Thompson puts it. Curious to see if customers will adopt it. I think it lacks two key value props of Amazon: uniform guarantees (Amazon guarantees regardless of seller) and super fast shipping.
BTW, when you say, "the buy has no recourse other than a refund if shipping takes longer than it's supposed to", do you imply that shipping via USPS gives you some other, more useful recourse if your shipping takes longer than it's supposed to? I cannot fathom what that would be, unless they're going to, like, refund your shipping and then give you $50 or something.
(Actually, if USPS was going to give me $50 every time they fumbled a package, I would ship everything USPS, always. I would order things I don't need, one shipment at a time. I would retire by the end of next year on my earnings.)
Actually, I've gotten free months of Prime with Amazon easily by just going into support chat and saying more-or-less, "You missed the guaranteed delivery date, I'd like a free month of Prime".
If this venue proves reasonably well functioning and promoted, big sellers will have to build their presence there, too, bringing in customers and more credibility.