It's usually pretty easy to find information about warranties on most manufacturer's websites:
"Only products purchased from an authorized Grado reseller will be honored under our warranty program. Grado resellers are not permitted to offer merchandise through any online auction services; hence any products purchased this way are not covered under our warranty program. When purchasing a Grado product please keep all receipts, for they will be needed to have warranty service performed."
This agrees with what you were told about buying the cable on ebay. I'd say your issue is with the reseller, not Grado.
https://gradolabs.com/company/where-to-buy
"Only products purchased from an authorized Grado reseller will be honored under our warranty program. Grado resellers are not permitted to offer merchandise through any online auction services; hence any products purchased this way are not covered under our warranty program. When purchasing a Grado product please keep all receipts, for they will be needed to have warranty service performed."
This agrees with what you were told about buying the cable on ebay. I'd say your issue is with the reseller, not Grado.
https://gradolabs.com/company/where-to-buy