recommended to overwrite at least 3 times just to stop a normal restorer proggy/disk editor not to be able to get much off it.
How much is the info on the hdd worth to you?
Is it more than the asking price?
if you haven't been conspiring to lead a group of mercenaries into guantanamo bay, or have the details of your last 5 years shiploads of cocaine, then i wouldn't worry too much