I have a late 2012 iMac i7 16GB RAM and 1TB FusionDrive which I want to sell. Using the Disk Utilities in Recovery Mode, I erased all data on the Macintosh HD partition. I then went on and selected reinstall MacOS High Sierra in the recovery mode, which failed a couple of times (no progress for hours) and after several tries it showed Mountain Lion (instead of High Sierra before) as the OS to be installed. I tried this as well, but the installation process froze after about 7 hours with "3 hour remaining". Then, I created a USB bootable installer for High Sierra. When booting the iMac and pressing the option key, I am presented this USB drive as startup option, and when I select it I end up in the recovery mode window again with the option to reinstall High Sierra. The installation takes about 5 minutes and in the very last second I receive an error message "An error occurred while preparing the installation. Try running this application again".
What options do I have from here?
Note: I already checked the system date, which is current. I checked if there's a recovery partition, and there is a Apple_Boot partition but without the usual name ("Recovery HD"). There are no disk / partition errors when running verify / repair.