Disk will not show up in recovery - iMac late 2015 - No disk error

Long story short: - I have a good working iMac 2015 5K that I wanted to sell. - I cannot do an automated factory reset on this device, so I had to go through steps. - I accidentally wiped a wrong disk, at least I think I did. - After wiping I get the folder/question mark at startup. - I restart with COMMAND + R - To get a Wi-Fi list to choose from, I connect and download certain recourses. - It reboots in a menu where you can go back to OS X (El Capitan). But when it asks on which disk I want to install, there is nothing. I cannot continue. - I did the first aid on all 3 given disks (HDD / SD and recovery) and it showed green, no faults. What should I do? Apple wanted to ship me off to a dealer, which costs me money. I know I did something 'wrong' but I need to get an OS back on my iMac. Any help is much appreciated.