It may be nothing to do with eBay. Sellers can choose not to deliver to particular regions in the UK. This usually involves remote areas or island, or Scotland.
Courier deliveries are more likely to be affected than Royal Mail, as couriers often surcharge for delivery to remote areas, which RM is not allowed to do.
The fact this has only just started suggests a connection with eBays new "simple delivery" service, which I think involves mostly courier deliveries? (I've never used it myself.)