You and/or the owner need to do this:
What to do before selling or giving away your Mac - http://support.apple.com/HT201065
You can erase it as suggested earlier, but depending upon the age there may be certain unpleasant things waiting for you. The owner may have set activation lock. The owner may also have set a firmware password.
About Activation Lock on your Mac - https://support.apple.com/HT208987
"What to do if your Mac starts up to an Activate Mac window
If you've purchased a Mac from someone other than Apple or an authorized Apple reseller and the device starts up to an Activate Mac window, the device is still linked to the previous owner's account. You'll need to ask them to remove the device from their account."
How to set a firmware password on your Mac - Apple Support - including how to remove a lock and what to do if you forgot your password.
https://support.apple.com/HT204455