Do you mean you do not remember your own username?
How do you normally reach your eBay account? Try using the same browser or app on the same device. Some browsers are set to remember form data or sign-in info.
Try signing out of your current eBay account first, or else use a private or incognito browser window to sign in to a different account. Click on the "email or username" field on the sign-in page to see if there are any suggestions.
If you cannot find any reference to your username by checking emails from eBay, try signing in to eBay using your email address associated with that eBay account in place of your username.
