Is this available via the APIs? I'm not able to locate it. Buyers can change their username and not only that, another buyer can take a previously used username after a certain period, so I can never be sure that a buyer with a given username is actually the same person over time.

Update:I found the answer and I'm posting it here for anyone who may be looking for this information in the future. I had to use the Trading API from the eBay Traditional Seller APIs. The GetUser call returns an EIASToken which uniquely identifies a user, even across UserID changes. For more information:https://developer.ebay.com/devzone/xml/docs/reference/ebay/GetUser.htmlhttps://developer.ebay.com/devzone/guides/features-guide/content/Development/CRM-UserInformation.html

Unique, immutable buyer identifier | Talk to your fellow developers

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Unique, immutable buyer identifier | Talk to your fellow developers | eBay Community