Anyone who sells a used hard drive, or a system containing one, should follow the step-by-step guide to using DBAN (Darik's Boot And Nuke), a great free program to securely erase everything, BEFORE packaging and sending the goods to an anonymous eBay or car boot sale buyer.
DBAN does a good job but overwriting the entire disk surface several times with random data is not a quick five-minute-or-less job - it may literally take hours to do thoroughly. Don't leave it to the last minute and don't cut it short if there is anything vaguely incriminating on the disk.
Oh and don't try this on any disk drive whose contents you actually still might need (doh!).