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Here is more information from Google about wiping a Chrome device:
There is a function called “Powerwash” in Chrome OS. Not only does it have a catchy name, it does exactly what you need. Go into SETTINGS, then select SHOW ADVANCED SETTINGS. Look down near the bottom of the dialog box, and you should see POWERWASH. Click Powerwash to “remove user accounts and reset your Chrome device to be just like new.” Restart your Chromebook, and it should be ready for you to sell.