Q: selling 2015 iMac

Restoring a computer like new for selling -  https://discussions.apple.com/message/24092964 - for computers that no longer come with DVDs and have an old AppleID associated with them

Apple: What to do before selling or giving away your Mac - http://support.apple.com/kb/HT5189 or  http://support.apple.com/kb/HT201065

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

https://discussions.apple.com/thread/5655535

Linc Davis Feb. 2016 post on restoring computer to new condition - https://discussions.apple.com/message/29726244#29726244

Before you sell or give away your computer, in addition to the steps listed in this support article, please take these steps:

1. Run Apple Diagnostics or the Apple Hardware Test. The buyer will do this (or he should), and you don't want to be surprised by the results.

2. Reset the PRAM and the SMC, which might contain personal information.

3. If you set a firmware password, remove it by running Firmware Password Utility in Recovery mode.

4. If you activated FileVault in OS X 10.7 or later, turn it off.

5. If you use Boot Camp, the partition must be deleted.

6. If you created any other data partitions on the internal drive, remove them in Disk Utility.

7. If the machine has an internal hard drive, erase the one remaining data partition with the option to zero out data. An SSD doesn't need to be zeroed.

8. Remove the machine from your list of registered products. If it's still covered by an AppleCare Protection Plan, transfer the coverage to the new owner by following the instructions in the AppleCare Terms and Conditions (under the heading "Transfer of Plan.")

Also note:

You can't legally or practically transfer any software downloaded from the Mac App Store to the new owner of the machine, even if it was free. That includes OS X, so if you upgraded to OS X 10.7 or later, you must reinstall an older OS by starting up in Internet Recovery mode (option-command-R at the startup chime.)

The new owner will have to redownload any software that came from the App Store, if applicable, including OS X upgrades, under his or her Apple ID. If you ever updated the bundled iLife applications (Garage Band, iMovie, and iPhoto) through the App Store, you can't transfer those either. The buyer will have to purchase them. Note that iPhoto has been discontinued and is no longer available for purchase.