No OS on iMac to Mojave

Hello,

Firstly thank you for looking at this post. I'm sure the answer is quite easy, if you know how, and I simply don't.

I am selling my trusty iMac of 7 years. I followed the instruction to erase all data from it and to restore the os back to Mojave. At some point something went wrong.

I have reloaded the os 3 times, every time I update back up to Mojave then erase, I lose the Mojave os and reverts back to no os! Then I Have to rebuild it from no os to through Mountain Lion and then to Mojave.

Each time the disk is erased, I have a 2 hour job to bring it back up to scratch.

I have tried to partition the drive and to keep the os on there, but my credentials are still there (password to log on and name (no iCloud data etc, as I don't set the iMac up with connection to iCloud).

Can anyone help?

Thank you,

Aidan