If nothing back after the ten day period, will definitely contact PayPal. Not sure what proof he could provide in his favor. The item was sold as new and evidence all seems to say otherwise - "N" model number, device/box label mismatch, warranty status/issues, mfg date. Plus, that it was correctly listed as an A1524 when A1522 was incorrectly printed on the supposedly unopened box seems to rule out a simple mistake (to me anyway).
Swappa allows feedback/ratings to be edited. So, I modified my original positive comment/rating accordingly. Initially, I had said transaction went smoothly, item as described, etc., etc. Now, I modified it to say things seemed good initially, but have now found item was resealed rather than new and in process of trying to resolve.
After 10 days PayPal should remove the money from the seller's account and give you a partial or complete refund. It's in the seller's interest to respond to PayPal or they'll lose the money and their product.