Are you able to sign in using a browser? If so, use that to keep track of what you are watching short term.
If not, the problem may not be with the app, it may be with your account, or your password.
Also try signing in using your email address instead of your username.
If you can successfully sign in with a browser but still not with the app, try deleting the eBay app from your device, restart the device, and then download the latest version of the eBay app from the App Store, and see if that makes a difference.
