The unfortunate thing is, items do get stolen if left in full view of passers by, it happens all the time.
Neither the seller nor eBay can be responsible where a courier / Postie leaves an item. If the seller has tracking to show delivery, they will not have to refund you.
Others have been successful with home insurance cover, you could try yours.
If this was a Royal Mail delivery, I would suggest talking to your Postie and ask if they please do not leave items in the porch in future, show them a safe place on your property or ask for it to be delivered to a neighbour.
@malza65