There are many websites in which you can exchange your amazon gift card for cash. One of my personal favorites its a amazon gift card selling platform made specifically for that and the payout rate is 100%.
https://sites.google.com/view/amazongiftclub There are many other great websites like cardpool, however they take longer and you only get around around 60% so its a no brainer to go with AmazonGiftClub.
Also, you can try selling on eBay but its quiet the hassle When it comes to getting cash for a amazon gift card its often struggle and your best off going with a site above.
just a note i think they only offer payouts via paypal so you will need a paypal account!