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I really wouldn't worry.
Reinstall the OS and zeroing out the HD should be enough, but not even necessary. Tools exist to scavenge drives, even after writing zeros, but they involve removing the platters and … Don’t worry.

They don't care, and the odds are that the person that gets your Mac after it's refurbed, will likley not care either.
 
What is it exactly that is that important, (before you jump me, I know some things are), that a erase and install wouldn't solve to a reasonable level?

Before doing this, how about sticking in Install disc 1, rebooting and opening up disc utility from the menu. Format from there then erase and reinstall.
 
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