Have you tried linking a PayPal account to your eBay business account? If you have not already done so then try doing that - you can set PayPal up to take money from a card, just as you would with eBay, or alternatively you can transfer funds from your bank or building society to PayPal so that if you wish to purchase something then the payment can come out of the PayPal balance if there are insufficient funds available on the card.
One other thing you should check when adding cards to your eBay account is that the name on the card matches that of the account holder, so if the cards belong to your girlfriend and the account is now in your name then that may explain why eBay will not accept the card. If, however, you have been attempting to add your own card to the account but it won't work then try opening a brand new business account (you would need a different e-mail address to link to the second account) and see if you can get things working that way when you attempt to add the cards to the account.
