There is software for this purpose. One I know about is Kill Disk... have not used it. Do a Google search for it and other disk cleaners. Find one that makes multiple passes to make the information unrecoverable.
Selling or otherwise disposing of an uncleaned hard drive is dangerous and risky. I would not do it.
Best of luck!
I was thinking of selling my laptop, but got a little bit scared after reading this
what software are they refering to and is it really free for personal use?