Well the back-end payment processing will have to pass the seller some sort of identifier with the purchase, at the very least something like an email address,

Well the back-end payment processing will have to pass the seller some sort of identifier with the purchase, at the very least something like an email address, so that you can have an account for that person. Then the seller will want to assign a license and/or customer number for their own records. If she doesn't know her email address or license, then yes she is screwed. Just like when I try to return things without a receipt. She can, of course, dispute payments with the credit card processor or whoever paid the seller. Even when a merchant provides clear paths to get refunds on their site, some people just prefer to do a chargeback which costs the seller a lot more than the purchase price.