If you pay for this hypothetical transaction with a bank card, credit card, or bank/wire transfer, there will be a record showing at least the transaction amount, the transaction date, and the recipient of the funds. Therefore, even if the reseller were to permanently close their doors, there will still be a bank account linked to them should legal action need to be taken.That's what I was thinking through. They might close, leaving a hypothetical buyer to evaluate the "receipt" and decide how else to authenticate.
We could postulate what-ifs ‘til the cows come home, but really, a receipt will be valuable only for you and your personal records. As with any ancillary item(s) accompanying a secondhand Hermès bag (i.e. CITES, clochette, lock, keys, dust bags, box, tissue paper, rain covers), receipts do not guarantee authenticity. Ancillary items must be authenticated separately. A bag must stand on its own merits. Period.