Even if the item is showing up as having been delivered I would still open an Item Not Received case and challenge it anyway, as even if the parcel has been delivered to somebody but it has not been delivered to you then clearly the item has gone astray at some point, in which case you would be justified in opening an Item Not Received case. Once an Item Not Received case has been opened the seller will need to prove beyond all reasonable doubt to eBay that the item was delivered to the address that you provided at Checkout, so if he/she cannot do so then you would stand a decent chance of winning the case and getting your money refunded.
In the event that you do decide to open an Item Not Received case but eBay decide the case in the seller's favour for some reason then it may be worth opening an Item Not Received case via PayPal if that was the method that you used to pay for the item, as PayPal tend to look at cases far more closely than eBay do. If, on the other hand, you did not use PayPal and paid using a credit or debit card then the best option would be to initiate a chargeback and see if you can get yourself a refund that way.
