Just bought a new 27" iMac and want to sell my 2007 iMac.

Question

I want to sell my old 2007 iMac after having just purchased a new 27" iMac. I have tranfered all of my data to the new machine but how do I now remove all of my personal details from the old Mac without losing to upgraded operating system? In the old days you would just re-install the system disc.

Answer 1

Before you sell or give away your computer, in addition to the steps mentioned in this support article, take these steps:
Run Apple Diagnostics or the Apple Hardware Test. The buyer will do this (or he should), and you don't want to be surprised by the results.
Reset the PRAM and the SMC, which might contain personal information.
If you set a firmware password, remove it by running Firmware Password Utility in Recovery mode.
If you activated FileVault in OS X 10.7 or later, turn it off.
If you use Boot Camp, the partition must be deleted
If you created any other data partitions on the internal drive, remove them in Disk Utility.
Erase the data partition(s) with the option to zero out data. An SSD doesn't need to be zeroed.
You can't legally or practically transfer any software downloaded from the Mac App Store to the new owner of the machine, even if it was free. That includes OS X, so if you upgraded to OS X 10.7 or later, you must reinstall  the original OS, either from the installation media, if applicable, or by booting into Internet Recovery (option-command-R at the startup chime.) If you installed from discs or a read-only USB key, deliver those to the new owner.
The new owner will have to repurchase the software, if need be, under his or her Apple ID. If you ever updated the bundled iLife applications (Garage Band, iMovie, and iPhoto) through the App Store, you can't transfer them either.
Remove the machine from your list of registered products. If it's still covered by an AppleCare Protection Plan, transfer the coverage to the new owner by following the instructions in the AppleCare Terms and Conditions (under the heading "Transfer of Plan.")