Trying to erase disk but "can't unmount"

Getting my macbook ready to sell and followed the directions here: http://support.apple.com/kb/HT5943

But when I try and erase the startup disk I get a failure message saying "can't unmount"

Will a simple reinstall of Mt Lion erase everything? If not, please help...

exact wording in dialog box is:

Disk Erase failed
Disk Erase failed with the error:
Couldn't unmount disk.

How are you trying to erase the startup disk? And which version of OS X is currently on it?

You can not erase a disk while it's in use. The error will come back exactly as you stated. You either have to boot the machine from a startup disk or if Lion, Mountain Lion, or Mavericks is installed, the disk can be erased from the recovery partition. In both situations, the disk would not be mounted.