To add one note: legally, you are not allowed to sell a Mac with an OS installed which was obtained at the app store because the license is not transferable and your Apple ID is tied to the purchase forever, so you are supposed to erase your drive and reinstall the original system; the buyer will/can upgrade/purchase any OS or app from the app store using their own Apple ID.
Thanks, I appreciate your input and I will do as suggested and advise what OS X is installed.
I have Time Machine on one drive, and CCC and other external drives.
What would be the best process to write zeros to the drive, use my CCC as boot drive and erase the drive?
I have done this before but want to make sure I don't miss anything.
One of two - in this case - would be to boot from your old install disk, wipe the drive and then reinstall the original OS on that disk.
Or, boot from a clone, wipe your drive and shut down the computer. Let the recipient install the OS from scratch. That method is great, but the buyer will not be able to "test" the machine.