Currently Being ModeratedNov 23, 2012 7:43 PM (in response to Yeukhon)
You cannot transfer your ownership of the 10.8.2 to a new owner.
Using the original disks that shipped with your iMac put the gray disk in and reboot holding the "C" key. Using Disk Utility erase/format your hard drive and then reinstall the original OS. If you have later versions (10.6.3 for example) you can sell those disks with the iMac but the downloadable 10.8.x is non-transferrable.
Currently Being ModeratedNov 23, 2012 9:32 PM (in response to Yeukhon)
Just as an added clarification: any OS license obtained from the App Store is forever tied to the Apple ID it was purchased with and is not transferable. If you left 10.8.2 on it, the buyer would not ever be able to download/reinstall because his Apple ID and the machine ID do not match the purchase records.
So you are required to uninstall the OS and reinstall the original OS - if that was indeed 10.5, and you do not have the original install disks, you will need to call Apple for replacement disks.