bought used iMac, previous owner wiped it clean

If you want to buy Mac OS X Mountain Lion on the app store, simply open the app store app (app store.app), find mountain lion (you can search), and click the "buy" or "download" button. This will ask you to sign in, or ask for your password if you already are signed in. When it finishes the download, you will be able to re-buy the OS on any computer using the ID.

If ML was installed when the first owner bought the computer, but now the internal disk is void, you do not need to buy ML again.

You can simply reinstall the OS through Internet Recovery see this document for details.

I hear that the installer for Mountain Lion is automatic. I hear that it opens automatically and deletes itself automatically as well. If you also want Mountain Lion, install it on your own computer and then download it on your father's iMac later, using the same ID. Otherwise, you can simply download it on your father's iMac. After that, you can buy it again on another computer using that ID.

Let me explain one thing: was that computer bought at the end of July? OS X Mountain Lion was released on July 20th, and that's a difference between if the computer came with OS X Lion or with Mountain Lion.

To avoid future problems, purchase OS X Mountain Lion on your Mac and, after purchasing it, start the used iMac, login with your Apple ID and see if Mountain Lion starts downloading. If it starts, just leave it until the installation finishes.

If you keep seeing the error message, download OS X Mountain Lion on your Mac and use Lion Diskmaker to create a bootable Mountain Lion disk. After creating it, connect it to the used iMac that you bought, press Option key while it's starting and select the external disk, so you can install Mountain Lion

In this case, download it on the Mac. When the download finishes, the installer will appear, so close it, download Lion Diskmaker and follow the steps of my last reply

I maybe wrong, but I think the original operating system from Lion onward is not connected with the Apple ID but with the Macintosh hardware.

You just need Apple ID to download the iLife suite, after installing the OS, that again, is free because it is sold with every new computer.

If you have created the bootable Mountain Lion disc, open Launchpad, press Option key and select X key above your OS X installer to delete it

Your post implies you do not have MacOS X M/L running. If it is a "new" machine originally purchased in July 2012 then it should have ML already installed. What happens when you start your machine?

If you bought M/L for anpother computer, you can logon to App Store with your AppleID and download and install it to the new iMac. You can then use your AppleID as the "admisntrator" account. Then create a new account for your Dad. Provided , of course, you are in the same household so as not to violate your user license agreement with Apple.

Lastly, if you restart your Mac in Recovery mode - hold down the Command and "R" keys simultaneously at start time - then you should be put into a mode to reinstall M/L. You'll see a menu come up giving multiple options for disk repair and maintenance.

Russa wrote:

If it is a "new" machine originally purchased in July 2012 then it shoudl have ML already installed.

But the OP has said that the computer was purchased in Early July, and Mountain Lion was released on July 20th, so it comes with Lion

.. so then he might have the 30-day free M/L upgrade installed.. actually it's too bad the original owner didn't just delete his old user account and leave an administrator account.. might be easier to contact the original owner about reinstalling Lion or M/L.

We don't know if the iMac was upgraded

.. so this thread might be read and provide options to the new owner.. afterall, the new owner ostensibly bought a working mac with an OS.. not a "wiped clean" Mac.