This is probably to do with Evri, one of ebay's chosen couriers. They charge more for outer regions plus Channel Islands and Northern Ireland.
Royal Mail charge the same for all these places so ask the seller to opt out of using Evri in their postage preferences and relist and you should be able to buy from them.
If they're not sure you can send them this link:
https://www.ebay.co.uk/ship/prf