If it's more than 90 days since delivery the seller would need to refund direct to your Paypal account, and would need your email address to do that.
However, if it's still within 30 days you can start a return on here:
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
select "Item doesn't match description" and if necessary escalate it to eBay after 3 complete working days.
If it's more than 30 days but still within 120 days of payment contact your payment provider and ask about a refund or chargeback.