If you still haven't found a solution to the problem by the time you read this response try doing what I did.
First of all delete the card from your eBay account, especially if you've previously used the same card to pay for an item that you bought from a seller who was opted into eBay's requirement for buyers to provide payment details when you make an offer, as opposed to after the seller has accepted your offer.
Once you have done this log into your PayPal account. If that card is your preferred method of payment then make sure it is linked up to PayPal as your default method of payment, although it is always useful to have a second card on file, whether it's a credit or debit card, just in case the first one can't be used for some reason - for example, if the card you use as your first choice to pay with is lost or stolen.
In my case once I had removed the card from eBay I contacted my bank and put a stop on the card by telling them that it had been lost. (The reason I did this was to prevent eBay automatically taking payment from it.) The card was then taken out of service and a new one issued to replace it. Once the replacement card came through I updated my PayPal account with the new card details and didn't bother adding the new card details to eBay. The end result of that is that when I make payment for something now I get a prompt at Checkout telling me to select a payment method, at which point I select PayPal and then pay for the item. It works every time.
