Mavericks downgrade to Mountain Lion - alert cannot be installed on this computer

I purchased a second hand iMac with Mavericks Developer installed already, seen as I am not a developer and other things don't work, I purchased a copy of Mountain Lion but when tring to run the installer it gives me this message "alert mac os x 10.7 cannot be installed on this computer" I want to keep all user profiles and software intact so I don't want to wipe the hard drive (although many other posts say when you get that message wiping the hard drive still doesn't solve it)

any ideas?

Verify that your iMac can run the older version of Mac OS X. You can see what versions can be run by searching for your specific model of iMac at EveryMac

Sometimes I have seen that even if a Mac was shipped with an older version of Mac OS it is difficult to downgrade due to firmware upgrades that make the system incompatible with older releases. I haven't personally experienced this sort of issue in maybe 10 years.