I have seem some similar complaints recently, but I do not seem to be having any problems switching between accounts myself, using another browser or a private or incognito browser window.
Make sure you are not using an old bookmark or a bookmark that has the username embedded in the link, such as a store link.
Try a private or incognito browser window. Or another browser. Preferably the latest version of a new browser that you have not used before, with no add-ons, and using default settings.
Try signing in using one of these links:
https://www.ebay.com/signin/
https://signin.ebay.com/ws/eBayISAPI.dll
If the username is already supplied on the sign in page, select "switch accounts" to change the username. If the option you want does not appear as a suggestion, pick any other option and then type the correct username over that option instead.
If you are able to sign in, you can avoid issues when switching between accounts by using a different browser for each account and staying signed in to each at the same time.
