I bought an Apple Gift Card and it says it's been used

See -> 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

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Card has already been redeemed

If you see a message that says that your card has already been redeemed, you might have already redeemed it. Sign out of the App Store or iTunes Store on all of your devices, then sign back in. This refreshes your Apple Account balance. Learn how to check your Apple Account balance.

If you need more help, contact Apple Support.

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

If you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.

Get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service 

Then you were scammed by whoever sold it to you.