Monkey - are you outside the US? A common problem is, that some shipping methods, that are trackable within the US are no longer, once they leave the country. People sending outside the US seldom know. At least from what I have experienced.
I (in Europe) have lost a knife thanks to USPS First Class Registered as well. I always ask for USPS Express mail now, as this can be tracked outside the US (at least to my country).
Good luck to you, and I would't automatically blame it on the seller. Chances are, he didn't know.