Theft of parcels from the doorstep is a soaring crime. The police in some areas are advising buyers to consider using click and collect services instead.
Once an item is tracked as delivered, eBay's involvement with the delivery ends. You can try opening an item not as received case, but if the seller can provide tracking proof that it was delivered you will not qualify for a refund. (You accepted this in the user agreement.)
That doesn't mean that you may not have any consumer protection rights. If the seller was a UK business seller they may be liable if their courier left the item in an insecure positon. But for some reason we almost never hear of this being successfully done, or even attempted. Probably because the work and uncertainty involved isn't worthwhile for the value of most items bought on eBay.
