If it was USPS First Class, I suggest you wait longer since you still want the item. Some items are still way behind and aren't showing movement for weeks, but aren't lost ... just delayed in certain locations. Talk to your seller and ask for any advice.
My advice as a seller this has happened to several times in as many months is to wait a reasonable time before having buyer file an item not received case, especially if they are willing to wait longer for the item. The reason for this is that usually right after you refund the buyer, the package usually shows up a few days later and there is no way in eBay to recharge the buyer. So they are on their honor whether they repay you for the item at that point.
I just had a first class mail package that had fallen into a tracking abyss and per request, I refunded the buyer after about 3 weeks, and after 5 weeks the package started moving again and was delivered. I've had several very slow to arrive packages, but haven't had USPS lose one yet either as a buyer or seller.
