If you purchased the RX6900XT on Ebay than most likely it came from a Scalper which means that AMD Warranty is automatically voided since the Scalper is the original owner and not you.
AMD Warranty is not transferable to a second owner this is also fairly standard for most 3rd Party Warranties.
I suggest you go through the Ebay Retailer about getting Warranty service or where to get Warranty service on the GPU card.
I saw that Ebay sold Powercolor and Sapphire RX6900XT. If that is one of the GPU Cards your purchased then you must go through Powercolor or Sapphire Support to get Warranty on it.
AMD Warranty only applies to GPU cards purchased from AMD Store directly.
Here is AMD Warranty concerning 3rd party GPU card purchased from Non-AMD Store Retailer: https://www.amd.com/en/support/kb/warranty-information/consumer-graphics-step2b