First, OP, i want to tell you how sorry i am that you are having to deal with this atrocity.
Secondly, as jaded and presumptuous as I may sound, I absolutely, unequivocally DO NOT BELIEVE that your "buyer ahem *thief*" received an "empty box"... while there is the usual devil's advocate who will chime in and say that jumping the gun and blaming the buyer is unfair, I will openly admit to being accusatory towards this buyer... but hands down, this has just become the oldest trick in the book, and out of all of the parties involved in this transaction, (from seller to USPS to your "buyer") the buyer is the person in this transaction who KNEW what was in the box, desired the expensive and coveted item, and has the seller and USPS at their mercy.
I would immediately contact the USPS and do every single thing possible with regards to an aggressive fraud claim, and let the seller know that under no uncertain terms, will you let this go without fraud investigations and reports to the fullest extent possible. This can be done without coming out and blatantly accusing the buyer, but to be frank, I am sad to say that another criminal has outright stolen via a horribly flawed system.
It just blows me away how many sociopathic, entitled and soul-less scumbag fiends get away with felonious offenses and think NOTHING of scamming, stealing, lying!!! omg WHAT THE

is WRONG with our society?!?!




Again, I am SOOOOO sorry OP, and please fight fight with all of your might against this criminal!!