If you really want to be totally sure and at the same time have the peace of mind, probably just swap out the hard drive. For your past files to be as totally as unrecoverable as possible(I'm not even sure if there's such thing as 100% unrecoverable), as far as I know it's probably going to be quite complicated that you might as well just save yourself the time and change the hard drive.
Indeed,the Hard drive is the most important part in this one,so better either swap the HD or buy new HD to put on your old Laptop before selling it so there are zero chance of being recovered.
Good for you that being aware of this because others just have sold their old Gadgets and they will be at risk.