Still not coming up for me either as a current listing or a completed listing.

That's weird. Here's a direct link:

https://www.ebay.com.au/itm/Analogue-Mega-SG-Cartridge-Adapter-Set-v1-1-Brand-New-Sealed-Free-Shippi...

All the seller's other items are being sent by the GSP so it may be that when the seller tried to change that on your listing he messed up somehow.

The seller is also currently trying to help me, so some things about the listing could change as this is going on. At the start it said it would post to my location, then for a while it said the opposite. Currently it says it does post to Australia (attached screenshot) but the checkout still says otherwise. The seller has never had this issue before, as they've sent items all over the world.
I've also tried changing my address' country to many other countries, which do work. It's just Australia that's acting up. How convenient.

Checkout screenshot.

The screenshots won't load for me, but nevertheless I suspect the seller needs to change the listing back to the GSP, then you may be able to buy it.

Or they could make a new listing of it for you to buy.

OK, it seems to have worked iitself out. The seller double-checked everything, that GSP was enabled and everything else was good and proper. Lo and behold, the checkout worked and I've now successfully purchased the item.
The solution? Absolutely no idea. The seller doesn't know what happened either.

It would be great to know what happened so I could post it somewhere and help others having the same issue.

Thanks to everyone in this thread for your help and suggestions. It's greatly appreciated.

Oh yeah, sorry for posting the question in two different places. I hope that didn't cause any confusion.