If it were me, I would not request to cancel. I would wait until one day after the estimated delivery window had closed and open an Item Not Received Case. If the Seller doesn't refund within 3-days ask eBay to step in and help. An INR protects you as a Buyer and dings the Seller's account for bad practices so protects others. You can leave Feedback to protect other Buyers too.

If they Seller does not have the item they should cancel for this reason. Again this will ding their account and help keep eBay running more smoothly for other Buyers. Good luck!