Hi. My name is***** you for your question and the opportunity to assist. Do you know your gmail e-mail address and password?
If you can't get assistance from the former owner you can also reset your Chromebook to its original factory state from the sign-in screen by pressing Shift + Ctrl + Alt + R and clicking Restart.
It's a lot of keys to push. You may find this helpful:
Hi. Were you able to reset the chromebook and get in?