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I understand if eBay suspects/finds an eBay account is taken over, all the user tokens associated with the account are revoked by eBay Customer Support.
However, why my API calls are still failing with an error message saying: “For security reasons, please log in again” or "Authentication failed : Token has been revoked by admin" when using the new user token generated after the account has restored back by eBay CS?
An Auth Token will be revoked by eBay CS if eBay finds the user account in question was Takenover (ATO), and the account owner will receive an alert email for the same. After the user account is restored by CS, the affected user can generate a new token and use it for making API calls.
if your new token gets revoked within a few hours again, chances are that there are a few things that may still need to be resolved. e.g ebay Customer Support may have detected an active item that was listed fradualently and therefore ended the item which can possibly trigger a token revocation.
Please reach out to the eBay CS team and work with them on ensuring that there are no other items or pending actions/ATO cases on your account that can trigger the token revocation.