it could be the postie put it in the wrong letter box? I used to work as a postie so it happens by mistake from time to time... normally the person who recieves it just returns it to sender or reposts it hoping it will get to the right address
It could also be that the buyer has the wrong address on paypal.. like they might have moved but not updated there paypal account... I have also done this before
Furthermore as a buyer I have had to chase up parcels, some have been returned to the buyer for no reason whatsoever, some where because of incorrect postage, some cause they where sent to the wrong address.
As a seller I have them returned to me cause they confused the sender and buyer... now I make a point of writing TO: in massive letters
I wont worry or assume anything but I would message the buyer to let them know as its properly a genuine mistake