I wish I could make places understand. If you do a single wipe with all zero's the ONLY way you are going to get any data off that drive is if you have a clean room and a professional setup (like a drive recovery company). I believe the theory is that if you wipe the drive with all zero's (or any other predictable pattern), you can still read the platters and there will be very very slight differences between each zero, and you can then determine which bits were originally one's. No one is going to go through all that trouble to steal a credit card number.
If you wipe 7 times with alternating and random patterns I believe it's been proven that you can not get anything useful off of such a drive.
Fortunately the places I get stuff from either don't care, or they just tell me to wipe it. One place pulled all the hard drives from G4s but left them in iBooks (too hard to remove). So they got less money for the G4s due to their condition.