There are two ways to prepare your 2012 Mac Mini to sell.
Make sure you are connected to the Internet. Restart using Command+R to boot into the Recovery partition on your internal drive. Go into Disk Utility and erase your Macintosh HD. Then back out of DU and select Reinstall OS X from the OS X Utilities menu. This will download Mountain Lion that came preinstalled on your system and reinstall it. After your system boots to the initial setup after ML is reinstalled, you can power off your Mini.
Second way is exactly like the previous paragraph only if you are unable to get into the Recovery partition (Command-R), restart using Internet Recovery (Command+Option+R). Then follow the other instructions above.
Command+R or Command+Option+R will get you to OS X Utilities (Recovery) so you can erase your internal drive and reinstall OS X.
Good Luck!