No, it's ebay doing it.
The seller has investigated this and changed the item to another category. UK then appears in the allowed country list.
It may be that the seller chooses to some extent, but ebay appears to control the global shipping list.
If it's any help to someone else you may find that your part that has no reason be covered by CE/UKCA legislation will not have it misapplied if advertised in
"Consumer Electronics>Radio Communication>Parts & Accessories>Coax. Cables & Connectors"
Note that ebay are quite wrong to do this : if the equipment was being shipped to the EU before 1995, or if it was after 1995 and it was shipped with CE marking, then spare parts will also have been certified. Accessories may not have been marked, but will have been if they were originally shipped with the item. UKCA grandfathers everything marked CE and will continue to do so even for new goods until december 2024 (and probably forever since it would be stupid to make the certification rules different).
There is a requirement on secondhand electrical goods sold by organisations such as auction houses to perform an electrical safety test (PAT). This is not related , however, to CE or UKCA marking. Conceivably it could be applied to personal imports but it seems unlikely, and CE marking wouldn't negate it if it were.