This probably belongs on Off-Topic, but, does he have a tracking number? It's HIGHLY unliekly that he would have shipped the mouse seperately, as its far more expensive to send 2 boxes rather than 1 slighly larger box to fit everything in. I'd say you got taken, so unless he can provide some sort of evidence that it got lost, I think you need to file a paypal/eBay claim against him.
Also, if it actually DID get lost, UPS provides automatic insurance on anything up to $100, so tell him you'd like to go through the claims process.