Can not redeem the card I purchased

This link will take you to the section of the support article about what to do if your card is not activated If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

See this Apple article:  If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card

If you can't redeem your Apple Gift Card or App Store & iTunes Gift Card - Apple Support

Beware of -> About Gift Card Scams -> About Gift Card Scams - Official Apple Support 

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I bought an Apple Card through eBay

You can report to eBay, see eBay Security Center -> Report a Concern.

• Need to resolve an issue with a buyer or seller? If you've already attempted to contact the member but were unable to resolve the problem, you can open a case at the resolution center.

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If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)

Where to buy a physical Apple Gift Card or App Store & iTunes gift card: actually, you can buy an Apple gift card at many places, at an Apple Store or an Apple Authorized Reseller in your region.