The sale was made and payment made before delivery, with the understanding that it would be delivered. The seller did not realize that she had a second floor apartment. I previously described what the UCC says about the seller taking possession for delivery for the sale to be complete and passage of title and risk of loss to transfer to buyer. If it was being delivered to a business on the second floor of a building and the seller agreed to deliver it, then seller is committing to deliver it to the second floor. Not sure if UCC applies to a Craigslist sale, but a Small Claims Judge might rule in favor of the buyer.