Trying to reset iPhone 5c, it's not recognizing Apple ID password?

Hi all! I recently upgraded to an iPhone 6s and was hoping to trade in my old 5c for some store credit at Vintage Stock or Gamestop. However, I am unable to "Erase All Content and Settings" because it is not recognizing my Apple ID password, even though it is clearly correct. I even tried changing my password and it won't accept the new one, either. My ID/password works on my new iPhone 6s as well as my iPhone mini, it's just the 5c that is suddenly not recognizing the password.

I have tried resetting all settings, as well as resetting my network settings, and it didn't help. Tried disabling find my iPhone and it prompts me for the ID password as well, and it still doesn't work. I am hitting my wit's end, so I'm hoping someone may be able to help. Thank you!