Nope. Nothing you can do. This is why most sellers demand registered post if using paypal, because it is trackable and can be proven. What a bummer.

I have recently been on the other end, and I 100% trust the seller that she posted the items. I was a little more understanding, I think, and asked her to re-post at half price, i.e. we split the loss. But that probably wouldn't work in your case, or with that buyer. Sucks.