The usual solution is two fold.
The seller needs to opt out of Evri as a courier option , and using RM is an added bonus as there is no extra charge to the Channel Isles, which Evri does have making the postage as you have found, extortionate.
eBay in their infinite wisdom have lopped Jersey and Guernsey into ' Europe' so if a seller has this as a place they do not post to, Jersey is then blocked. The seller here, needs to untick the box in their Site Preferences.
@coedpi