sorry to hear.
PP will tell the buyer to send it back under either an eBay item not as described OR a PP dispute. the buyer has 2 options and 90 days. in fact, the buyer can return it up to 90 days for ANY reason. even if you state that you don't accept returns.
(from my own experience);
1) eBay will tell the seller, even without you knowing, to simply send the item back and they'll refund them with your funds in your account. (they can send a rock in a box and as long as its sent, ebay will assume they sent you the item.)
2) the refund ONLY happens IF you accept the delivery. with OR without a signature at deliver. UPS or FedEx or whatever can simply drop it off at your house and as long as it says it was 'delivered' the seller gets the money back.
3) WHATEVER you do, DO NOT open it unless you have a witness and a video recorder going. better yet, DO NOT accept it if you're there when it arrives. tell the driver you do NOT accept it. (the seller doesn't even need to post a tracking # to keep you in the loop.)
4) communicate ONLY via the resolution center message section. NO personal emails or texts. eBay will rely on their message center notes because any case worker can answer the call and they always need a few minutes to read it when you need their help.
this is a typical worst case eBay PP buyer fraud scenario, its rare, but when it happens, its a kick to the gut. without prejudice; if what you're describing is absolutely accurate, i really feel for you and know how you're feeling.
Hope it works out.
