Please follow this step:
Format your system and reinstall the original operating system software that came with it. While you can install the latest version of OS X that you purchased from the App Store, technically this is a copy licensed to you, not the new owners. Reinstalling the original version of OS X will allow the new owners to choose what version they would like to install.
To reinstall the original version of OS X if your Mac came with a gray installation and restoration DVD, insert this into the optical drive and reboot with the C key held down. Then after selecting your language choose Disk Utility from the Utilities menu. In Disk Utility, select your internal hard drive (listed above any volume names contained on it), and you should see a "Partition" tab appear. In this tab, choose "1 Partition" from the drop-down menu, and then give the partition a name, set it to be "Mac OS Extended (Journaled)," and apply the changes.
If your system did not come with a gray restoration DVD, then you will need to use Internet Recovery to restore the original version of OS X. This will access Apple's servers and download the installation tools for the correct version that came with your system. To do this, ensure you have an appropriate internet connection
for use with Internet Recovery, and then reboot with the Option-Command-R keys held to force loading to Internet Recovery. Again, use Disk Utility to fully partition the internal boot drive, and then quit Disk Utility.
Once the hard drive has been formatted, you can proceed with running the OS X installer, and wait for the content to download and be set up on the system.
After the installer completes, the system will restart and show the welcome screen of the OS X setup assistant, at which point you can hold the power button to shut off the system (when it is next turned on it will start at the assistant). It is now ready for the new owners to turn on and set up, as if the Mac arrived straight from the factory.