If you want to go back to square one, re-format the hard drive. You will have to reload the operating system which, if you don't have a licensed copy of a disk, may cost more than you will get for the laptop anyway.

If you are confident you have all your files deleted and want to make them a little more difficult to "undelete" do a "defragment" of your drive.

This will move file fragments around and overwrite a lot of deleted files still residing on disk space.

If you want to protect your files from a high grade hacker, don't sell the laptop.