Transferring Mountain Lion
You can only share it if you bought it from the App Store.  Pre-installed versions are single-copy licenses: 


  A. Preinstalled and Single-Copy Apple Software License. Subject to the
  terms and conditions of this License, unless you obtained the Apple
  Software from the Mac App Store or under a volume license, maintenance
  or other written agreement from Apple, you are granted a limited, non-
  exclusive license to install, use and run one (1) copy of the Apple
  Software on a single Apple- branded computer at any one time. For
  example, these single-copy license terms apply to you if you obtained
  the Apple Software preinstalled on Apple-branded hardware.


https://discussions.apple.com/thread/4971852?start=0&amp;tstart=0