Here's yet another article on data recovered from hard drives sold on eBay. I have a better recommendation for getting rid of your old computer equipment: consider not selling your hard drives (including the ones in your printers). Instead, dismantle them and break (or grind) the platters.
Just for info, the platters are engraveable. They make very nice geek awards for departing sys admins or employee of the month (if they fill a geek role).