Why would ebay change my name, from the one I've had for years, to something non-plussed?

[sandypebbles]

You would need to ask them

I have never known eBay to just automatically change someone's ID

Are you the only person who uses the account?

Does someone else use the computer/device?

*Edited to add, the account you are using now has been open less than a year so you have not had an ID on what you are signed in on now  'for years'