Desperate need of OS help. mac ruined!

Hi, so basically i think i have wiped my OS from my mac mini. i am selling it tomorrow and tried wiping all of the time machine back ups. i deleted a partition and i cannot get past the flashing ? on start up. it wont even let me go to the start up menu by holding option key or anything. please please help! thanks you in advance.

If you had OS 10.7 or higher installed previously. Press the command + r keys when rebooting the computer...then reinstall the OS from there.

- Nick