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Those old gift cards were processed by PayPal. Around 2015, eBay moved giftcard processing back into eBay, but it is not clear whether the old card balances remained or were moved. You might go to your PayPal history to see if the number was ever assigned there and if it has a balance.
The only other thing I can suggest is that you enter that number when making a purchase on eBay. If eBay can find the number, they should process it. Otherwise, they should show an error with the card number. eBay gift cards never expire.
