How do I transfer ownership of Mountain Lion and iMac?

I'm assuming you're selling the iMac? What OS originally shipped with the system? If it did not ship with Mountain Lion, then the answer is no, you cannot transfer ownership of the OS. Your copy of Mountain Lion is tied to your Apple ID. To sell the computer, you would need to erase the hard drive and reinstall the version of the OS that it, originally, shipped with. The new owner would, then, need to purchase their own copy of Mountain Lion.

Excellent suggestions re. selling - if, on the other hand, you purchased it and it did not originally come with Mountain Lion installed, then you will not be able to transfer the license as it is not transferable. You will need to get the original OS install disks or, if it came with Lion, then you can try using Internet Recovery (Command + Option + R during restart) to get to Apple's servers. Once there, use that Disk Utility to erase the drive and it should reinstall the original OS (whatever version of Lion). You can then purchase your own license of ML using your Apple ID.

dcb0 wrote:

Are there any administrative things to do, such as registering the transfer of ownership from one person to another. Is there a way to indicate that the OS and apps have been sold so that they are transferred from one Appleid to another?

There is no possiblility to transfer from one AppleID to another.

I don't know if there is a capability to retrieve the originally bundled apps from the App store. I would imagine it is not possible.

What Barney said...anything you've purchased through the App Store belongs to you and cannot be transferred. Such items should be erased from the computer prior to resale.

There are a number of things that should be done to, properly, prepare a Mac for resale. Here are a few articles that explain the crucial steps in more detail than I possibly could:

http://www.tuaw.com/2010/09/24/mac-101-preparing-your-old-mac-for-sale-or-recycl ing/

http://www.thesafemac.com/how-to-prepare-your-mac-for-sale/

http://reviews.cnet.com/8301-13727_7-20011109-263.html

I would also suggest that you secure erase your drive.

When reformatting, in security options, select the value

to write a single pass of zeroes to the drive. A simple

format only removes all the directory settings, leaving the

actual data still on the disk. An unscrupulous person could

still recover that data and possibly compromise any personal

information left on it.