Purchased cards are already used

Please follow the steps in this Apple Support article. 

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

If after reviewing the article, and following the directions in the article, you need additional assistance please follow the link below to contact official Apple Support. 

Contact Apple Gift Card Support

If you receive a request for payment using Apple Cash, or If you believe you are the victim of a scam, please contact Apple Support and report it to local law enforcement.

https://support.apple.com/en-us/120933

You may also report the scam to the FTC Complaint Assistant and to the FBI’s Internet Crime Complaint Center

If you receive a request for payment using Apple Cash, or If you believe you are the victim of a scam, please contact Apple Support and report it to local law enforcement.

In addition to the information Jeff Donald provides, and in addition to the type of scams in the support article at https://support.apple.com/120933, there is a special scam for cards purchased from a store. The card was already compromised before you purchased it. I personally would avoid buying a gift card in a store unless it was stored in a secure location to which only sales personnel have access. I would also be inclined to avoid purchasing any high-value gift card.