At least here in the U.S., I'd wait until the "Expected Delivery Date" notice appears in the posting. It should (again, at least here) have a Tracking Number associated with it. If, as the date approaches, you haven't seen any signs of posting, I'd send a semi-friendly reminder and ask if there's a problem. (I had just such a case a few weeks ago and the seller, after a few days e-mailed that a family emergency had taken them away from home, apologized and assured the item had been shipped... it arive within a couple of days).
After notifying the seller and, especially if there's no response, then I'd open a case with E-bay. They've been very helpful and brought to resolution every case I've opened...