Bought Used Mac how to Reset?

What to do before you Sell, Give Away, or Trade in your Mac

Shut down your Mac, then turn it on and immediately press and hold these four keys together : Option, Command, P, and R. Release the keys after about 20 seconds. This clears user settings from memory and restores certain security features that might have been altered.

How do you Change Ownership of a Mac?

Create new administrator account in System Preferences > Users & Groups. ...
Log out of current user and login with new admin account.
Change the name of the original users home directory /Users/original to the new desired short name /Users/new

How do I Delete Previous Owners on my Mac?

Select System Preferences from the Apple menu, click the Accounts icon, click the Lock icon, and enter your password.
Select Logout from the Apple menu.
On your new computer, triplecheck that all the files for all accounts on the old machine have been transferred.

How do I Reset my Second Hand MacBook Air?

Hold down the Command and R keys on the keyboard and turn on the Mac.
Select your language and continue.
Choose Disk Utility and click Continue.
Choose your startup disk named Macintosh HD by default from the sidebar and click the Erase button.