Option 1) use a USB stick to boot from. There are more than a few recovery tools, and even full blown linux live images, that you can run off a USB stick. Run a factory format of your hard drive about 7 times to prevent someone from trying to recover any data
Option 2) Sell it without a hard drive.
Option 3) Take your Hard Drive out back and smash it with a hammer. Nobody will be getting anything off of it then.