I'd start by going into your post office and speaking with your postmaster immediately. Print out/screenshot the various tracking information (eBay, parcelapp, etc - try just straight entering the tracking as a number in google and see what comes up) that you've been able to find and ask them what this possibly could mean. The tracking still HAS to still exist somewhere in the USPS system. You deserve a better answer than "It doesn't exist in our system" when the last tracking showed it at Customs in California. It'snot like it never scanned into the USPS system, ever. If that gets you nowhere, track down the regional manager.
I'd also be escalating this with eBay - you deserve a better answer than "you cancelled the shipment" when they could see that it was in customs in Cali - it's not like the tracking shows it back with you or on the way back to you. I've never tried to get something back that has already logged into export customs, but I can't imagine that is easy to do. Press them on whether it shows cancelled based on information from the USPS tracking or if someone logged a cancellation of the label in the eBay system.
Which does lead me to the following questions - is there any way the item was deemed ineligible for export? Stuff does get coded wrong - at least once a quarter I get a outbound package scanned as "delivered" when the clerk neglects to check the scanner. Apparently the USPS is so archaic that they cannot correct a scan 🙄. Something deemed ineligible for export does not automatically come back to you - you've got to go through lots of paperwork and hoops to maybe get it back. I speak from experience.
And as frustrating as this is - depending on the value of the item, there does become a point in which you are spending way more time than the item is worth to chase it down. Your hours have value.
