Thanks.
Looks like it’s just unlucky if you happen to live in Jersey.
Oddly enough, eBay applies the correct UK postage for Guernsey without any issues, but Jersey gets lumped in with the rest of the world for some reason.
Oh well—100,000 fewer people to sell to.
I suppose the problem will quietly die away in a forgotten corner of eBay’s backend, never to be fixed by whoever programmed it wrong in the first place.
I’ll report it anyway, for all the good it’ll do.