Buying and selling used Mac

iMac10,1 is the Late 2009 iMac, so it came with DVDs. I strongly recommend you to erase the drive and reinstall the OS X version that came with it (Mac OS X 10.6.1 or 10.6.2), so the new owner can use it legally. Call Apple to get replacement DVDs > http://support.apple.com/kb/HE57

After that, insert the Mac OS X Install DVD and hold the C key while your Mac is starting. Choose your language, go to Utilities menu (on the menu bar) > Disk Utility, and erase the drive > http://pondini.org/OSX/DU1.html

Finally, close Disk Utility and reinstall OS X. When the reinstallation finishes, hold the mouse while your Mac is starting to eject the disc and turn it off. Tell the new owner to use the Applications disc to install iLife applications after setting up the Mac

The Mac should be sold on with the original OS re-installed. If you've upgraded to the latest OS that is assigned to your AppleID, not the Mac, and is not transferable.

What OS was installed on it when new? Lion or later and there are no disks - erase and re-install of the OS are done via the Recovery HD:

http://support.apple.com/kb/HT4718

If it had an earlier OS (Snow Leopard perhaps) you need to find/replace those disks (Apple will sell a replacement set for a modest charge). Boot from the disk, erase and re-install the OS. The disks should then be passed on with the Mac to the new owner.

Obviously, ensure you have copied off any data you want to keep before doing the erase/install 😉.

Paul, if the OP has got a Late 2009 iMac, it came with Snow Leopard. Just to make this clear to the OP ;-)

Thought so, that's why I included it in my post 🙂.

Don't sell it with OS X Mountain Lion or you will have problems.

OS X Mountain Lion is registered to your Apple ID or the Apple ID of its old owner. If you give the iMac with OS X Mountain Lion to the new owner, he/she won't be able to reinstall OS X unless you give him/her your Apple ID email and password, so it's not valid. You have to reinstall Snow Leopard.

You can get the replacement DVDs by calling Apple > http://support.apple.com/kb/HE57 Don't buy an used Mac with OS X Mountain Lion or Lion unless the owner has deauthorized it

You won't be able to reinstall Mountain Lion if something goes wrong. That's the problem, and it's serious. There are a lot of users who came here desperated telling us that their Macs were useless because of that reason. This can only be voided if you bought Mountain Lion with your Apple ID

Of course, for a common use, you won't have any problem, but you will if the hard drive gets damaged

The purchaser of the used Mac is the one who will potentially end up with the problem if the Mac has the wrong OS installed. If anything goes wrong and a re-install is required they'll be stuffed. As has already been mentioned, the Mac should be passed on with the original OS installed and the new owner will need to purchase Mountain Lion if that is the OS they want.

Correct, but you won't be able to reinstall OS X if you haven't purchased OS X Mountain Lion

Much better. You can be sure they will work