I just had a situation where I bought an amp on Reverb, it got on a truck in Kentucky and basically went straight to the Mail Recovery Center in Atlanta. I found out about that because I found the amp on eBay after about two months, while my claim still hadn't been paid. The seller claimed he gets his stuff from the MRC and he got seriously pissed when I mentioned the USPS Inspector General. This news segment tells you where lost packages from UPS and Fedex go, maybe it can help you in following up. I certainly don't believe the amp I bought was "lost"
the seller got the amp from you got it from MRC? That is shady. How did he get it? That FedEx hub in/outside of Atlanta is a black hole of bewilderment. Avoid at all costs!!