Unfortunately, some couriers / posties leave items in not so safe a place, and if it view of passers by they can and do get stolen.
If the seller has tracking to show delivery they are not liable for any refund, as they are not responsible for where an item is left.
Have you asked neighbours if by chance it was handed to them? If this was Royal Mail, ask your postie where they left the item.
@pennicrawfor-0