Adding prepaid cards to Paypal?

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Paypal says that you can use prepaid gift cards to pay for things. I tried adding a prepaid gift card while checking out from ebay. But for some reason the payment didn't go through. The item I was buying was less than the amount of the gift card, and the gift card itself is already registered in my name. Does anyone have any suggestions on why it didn't work? I could have called Paypal, but it was too much hassle over one little item, so I just used my regular payment method. But for future reference, I would like to know why it didn't work. The same card later worked fine in Amazon to buy a gift card.