I used to sell a lot on Half.com, and I always did wonder what would happen if the buyer never got the item, whether they were truthful or not. Especially since the standard shipping method is Media Mail, which can take up to month or so to get there. Not to mention they don't tell you to use insurance nor do they reimburse you for it. Thankfully, I haven't had any real problems yet.
Quote from Half.com:
Currently, Half.com has Buyer Seller Communication set up so the buyer can report a problem to a seller, and the seller can respond with their side of the situation. After the seller replies, no further communication can occur directly between the buyer and seller.
Seems kind of limiting, with only once chance to say something...
I have had experience with DC though... I would ask the buyer to call up or go to his local PO with the DC tracking number, and see if they still have the package lying around, or if they have any idea about. A buyer from me on ebay did this and got the package finally. Keep us updated...