I'm very sorry, but all the seller has to prove is that the item was shown by tracking to be delivered to your address, the seller cannot be responsible for where your postie / courier leaves the item. Sadly, some do leave them in unsafe places for all ' eyes' to view, and items do get stolen. The matter can be taken up with Evri, though I understand with some difficulty!
You cannot in this instance receive any refund from eBay.
You may report this to the Police, not for any real hope of retrieving your item, but they will record it, and it may show a spate of thefts in your area, for them to be aware of.
May I ask, when I take deliveries from Evri, I have one pending now, I always receive an email from them, to suggest a ' safe place' / neighbour's address if I wasn't going to be in, do you not receive such notification?
@hunnybunzz