A simple magnet is not sufficient to erase data. It will change the direction of the magnetic moment of the disk, but the with the right tools individual bits still have enough memory to reconstruct the original data. What you need is a degauss tool which will cycle the magnetic field direction a hundred times over and completely randomizes the bits.
Thanks for all the replies. I'm convinced:
1) Don't sell a used hard drive that ever contained "personal" or "sensitive" information.
2) Don't just throw it in the trash.
3) Rewriting it isn't guaranteed to make it unreadable.
4) Thermite, sledgehammers, or firearms will probably render it unreadable.
I'm opting for the sledgehammer approach.