Hard to say because we have no access to your account, but a few things you can try.
Ask the seller to put you on their buyer' s exemption list. Send them this link:
https://www.ebay.co.uk/bmgt/ExemptBuyers
They won't do this of course if you have had 2 or more unpaid strikes on your account and are on their list already, this would prevent you paying.
Ensure your cards have more than 6 months validity and the correct ones are registered here:
https://ppcapp.ebay.co.uk/myppc/wallet/list
@je-432101