eBay published the following post on the eBay UK announcement board about lithium batteries commonly used in consumer electronics:
Lithium batteries have been classified as Restricted Goods by the International Civil Aviation Organization (ICAO)
Last year, the ICAO reclassified lithium batteries, also known as power banks or portable battery packs as restricted goods, meaning that Royal Mail is unable to deliver them.
Any sellers on eBay.co.uk listing such items should update their listing to use an alternative carrier. Otherwise, these listing may be removed.
Find out more about prohibited goods on the Royal Mail website (link).
Source: eBay UK Announcement Board Post