I mean i ordered a new 2013 iMac and my 2012 model i want to sell it because i dont need them both. So what should i do with my OSX? Shall i restart and enter in disk utility and erase the fusion drive and that it? If i do that the next user can install ML by himself for free? Since this iMac is late 2012 and came with ML preinstalled.
Do a command-option-r boot (hold all three keys at once). That will start a recovery screen. In that screen start Disk Utility and use it to erase Macintosh HD. Then quit Disk Util and click install OS. Now wait while the 4.7GB installer downloads and installs. At the end a setup wizard will start to select language etc.... when that happens do a command-q to quit and shut down the machine. Now when the new owner starts it up they will get that setup wizard like it was a brand new machine. The Mountain Lion install is tied to the machine's serial number, so the new owner can repeat what I just described if they want to and get the OS.