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Can you tell us what model your macbook is and do you still have the disks for the machine?
The best thing you can do if you want to clear out your personal files and make the machine useable for a new owner would be to just reload the machine from scratch, That way all your personal files, passwords etc will be removed and the new user would then be able to setup their own choice of password, logons etc and never have access to yoru info.
To do that can you put the OS disk into your machine then press and hold the power button down to shut down the machine, Next can you wait 5 seconds then power it back up and as soon as you let go of the power button can you press and hold the C key down till the Apple installer language screen opens.
Just follow the prompts and it will erase then load a fresh copy of your OS to the machine.
Let us know if you hae any issues or if that does not work.ThanksEric