Selling an Old Mac:
• When selling an old Mac, the only OS that is legally transferable is the one that came preinstalled when the Mac was new. Selling a Mac with an upgraded OS isn't doing the new owner any favors. Attempting to do so will only result in headaches since the upgraded OS can't be registered by the new owner. If a clean install becomes necessary, they won't be able to do so and will be forced to install the original OS via Internet Recovery. Best to simply erase the drive and revert back to the original OS prior to selling any Mac.
• Additionally, upgrading the OS on a Mac you intend to sell means that you are leaving personally identifiable information on the Mac since the only way to upgrade the OS involves using your own AppleID to download the upgrade from the App Store. So there will be traces of your info and user account left behind. Again, best to erase the drive and revert to the original OS via Internet Recovery.
Internet Recovery:
• In the event that the OS has been upgraded to a newer version (i.e. Lion to Mountain Lion), Internet Recovery will offer the version of the OS that originally came with the Mac. So while booting to the Recovery Disk will show Mountain Lion as available for reinstall since that is the current version running, Internet Recovery, on the other hand, will only show Lion available since that was the OS shipped with that particular Mac.