Other people may know more than I about the process as I have never had anything like this happen, but I think you might be best to escalate as soon as you are able to.

The seller will not send any empty box without tracking, there would be no point. Sellers have to have tracking to prove delivery.

After you get all your money back from paypal or ebay, go to a proper shop to buy your Macbook. Ebay isn't the best place to buy computers or expensive things like that. Not saying all sellers are scammers at all, but you have to be very careful with expensive items. If they seem suspiciously cheap, don't be tempted to buy.