We received an email a few days before we left from the buyer, saying that he hadn't received the package. We checked the tracking and it said that the parcel was 'progressing through our system'. We rang the Post Office and they said that there was absolutely nothing they could do until Dec 12 (why that date, I don't know). So we let the buyer know of this, apologised for the hassle and kept our fingers crossed. In the meantime, we packed up our flat and left the UK on December 11.
The buyer isn't happy and opened a claim against us on eBay on the 12th of December, even though (according to Royal Mail) that was the first date we could do anything. Prior to that, we didn't necessarily want to give him a refund if there was a chance that the GPS would still turn up and he'd have both the GPS and the money.
This is all very shitty and annoying, especially since we haven't had any other trouble with Royal Mail all year and all other eBay packages sent at the same time arrived to their buyers with no trouble at all.
But the best part? The online tracking now says that the package is being 'returned to sender'. After almost a month, to an address that we no longer reside at, in a country we no longer live in. Thanks Royal Mail, that's super helpful.