Delete all personal files , remove any programs you may have installed except operating system, then do a defrag.
No, no, no and no. With all due respect, that's a terrible piece of advice.
Hard drives are cheap as chips. My advice where possible is to remove and keep "your" hard drive and then either leave purchaser to buy a new drive, or fit a new one for them.
Obviously, with laptops this can be a bit of a fiddly option.... so as others have suggested, try an appropriate tool.... such as the well named "Darik's Boot and Nuke".
Darik's Boot And Nuke | Hard Drive Disk Wipe and Data Clearing
P.S. gg.... I think you meant to suggest 'disk wiping' ...at a glance, your link seems to be almost as good as Avtrician's advice !! i.e. your tool only seems to "securely" delete chosen files on a booted drive rather than trashing the drive.