Did you Mac come from the factory with Mt Lion Pre-Installed? If not you can not sell it with Mt Lion. you have to reinstall the original version of OS X that it came with from the factory.
All other download versions of OS X are Tied to Your Apple ID and are Non Transferable.
Doesn't matter how you upgraded it to Mt Lion it is still an Upgrade and tied to your Apple ID. You Must Re-Install Lion and sell it that way.
To reinstall Lion you use the Online Internet Recovery system. to get to that at startup you HOLD DOWN the Command+Option+r key until you see a Spinning Globe in the center of the screen. From the screen that loads you use Disk Utility to Re-Partition the drive as One Partition. That is Very important, you must Re-Partition to get rid of the Mt Lion Recovery HD partition.
Then when that is done exit DU and Select Reinstall Mac OS X and Lion will be downloaded and install on the drive. At first startup after Lion has finished installing just hold down the power button until the system turns off. When the buyer turns it on for the first time it will be like it has been turned on for the very first time, going through all the setup screen just like it was New.