If you offer PayPal, you can automatically accept card payments via eBay checkout - a PayPal account is not required for the buyer, they can normally just chekout by entering card details, though the seller does ultimately receive the funds to the PayPal account.
If the buyer can not check out with their card, and your PayPal account is all set up correctly (including the ability to accept card payments), then it would be an issue on their side which unfortunately you wouldn't be able to resolve by doing anything your side. (If you haven't upgraded to a premier PayPal account, this should not affect your buyer's ability to pay by credit card outside of a PayPal account, normally what happens is they can pay and then the seller receives an email prompting them to upgrade and accept the payment, but it's worth double-checking if you just have a personal account).
If the issue is on the buyer's side, someone may be be able to troubleshoot and help resolve, but they'd need to post all the details in a new help thread.
