Which model do you have?
You may restore to factory setting to clean everything.
And please contact the seller to double check on the account issue.
Thank you very much.
search "control panel" in search box, then click on the "control panel" then choose view by "small icons" , then
click on "user accounts", then click on "manage another account" if you see another person's account means ,, delete it including user files.
This is Chromebook.
Sorry then ,no more ideas from my side.