Because you had the estimated delivery time available to you when you committed to buy the item, you agreed to the terms including that estimated delivery time (whether you actually took the time and effort to read it on the listing or not--the one estimate includes both the seller's handling time and the carrier's transit time), and the seller has that long to get it to you. You can open a case (or whatever the eBay PR types are calling it these days) once the estimated delivery date you got when you paid has fully passed, and then the seller has 3 days to respond before you can "Ask eBay to step in."
