A buyer from half.com received my package but it was empty. Do I owe him a refund or is this a scam?

I sold a book over half.com, and a week later the buyer emails me saying he received the envelope but it was empty. He asked for a refund and told me that "I could call the post office and see if they have it but he doubts they will". I don't think there's any way to verify if he's trying to scam me or not, so I don't know if I should refund the cost of the book. The envelope I sent it in was new and sealed by adhesive, and the post office isn't being cooperative. I'm also afraid of receiving a bad review on my Half/ebay account because I just joined and I don't have any ratings yet.