Did the machine originally come with Mountain Lion?
If yes, then try the recovery mode again to boot into.
If not, you cannot sell it with Mountain Lion installed because the license is tied to your Apple ID. The buyer will never be able to reinstall.
So, if it did not come with Mountain Lion, which OS was installed? Did you have install disks?
If so, that is what you need - insert the disk and hold the C key while hitting the Power key. It should boot from the disk. You can then go to Disk Utility to erase the drive and reinstall the OS.
If you no longer have the disks, call Apple, give them your serial number and ask for replacement disks.