While PayPal's checkout site can usually offer a card-based payment option for buyers who don't have a PayPal account, that will not allow paying with a card that is already linked to a PayPal account. The buyer should be able to go through checkout by logging into her PayPal account that has her desired card linked to it, rather than trying to pay with that card directly.
Aside from that general tip, we would have no insight from our end about why PayPal declined any particular payment. If you need further clarification, we would suggest calling PayPal at 888-221-1161, which should get you to an elevated tier of support better acquainted with E-junkie than their regular front-line customer service agents.