One thing that you ought to bear in mind, in case you are not already aware of it, is that when eBay provide estimated dates of delivery they often apply unrealistic dates to sellers' listings, thus giving buyers the illusion that they are likely to receive the item a lot sooner than may realistically be the case. Therefore, whenever I purchase anything on eBay, I tend to take this into account, and if an item does not arrive by eBay's latest estimated delivery date I will wait an extra five working days from eBay's latest estimated delivery date before making a decision to open a case. In the vast majority of cases whereby an item has missed eBay's latest estimated date of delivery it has still turned up within an acceptable timeframe, often arriving a day or two later. Perhaps I've just been fortunate, but there haven't really been too many cases where I've had to open an Item Not Received case against a seller - indeed, I've only had to take this course of action against one seller so far this year, and that was the first time I'd had to open an Item Not Received case against an eBay seller for quite a few years.
In view of the above I would advise you not to be too hasty with regards to opening a case just yet. Give it a little longer, and if you still have not received the item seven days after eBay's latest estimated date of delivery has passed then I would suggest that your first course of action would be to contact the seller via eBay Messages, explaining the situation and asking when you can realistically expect to receive the item. If the seller is helpful and makes an effort to resolve the matter amicably then hold off of opening an eBay case, as if the item subsequently turns up then there should not be any need to open a case unless the item is not as described in the listing.
In the event that the seller is uncooperative and/or obstructive then once the seven days have passed since eBay's latest estimated date of delivery, then if the item has either not arrived, or has turned up but is not as described in the listing, then open the appropriate case against the seller. The seller will then have three working days to resolve the matter without eBay having to get involved. If the seller subsequently sorts the matter to your satisfaction, whether that's by sending you a replacement item or issuing a total refund, then there's no need to take the matter any further. If, on the other hand, you don't seem to be getting anywhere with the seller then on the fourth day from the date of opening the case you should see an option to escalate the case to eBay. If it does become necessary to escalate the case to eBay so as to get them to step in and make a final decision upon the matter then you will more than likely receive a total refund of the money that you paid to the seller, so it's not as though you've got no chance whatsoever of getting your money back in the event that the item never arrives, or turns up but is not as described in the listing, and the seller proves to be unhelpful.
