Let’s assume you hate Mavericks and you want a new fresh installation of Mountain Lion. You can do that from Snow Leopard and Lion.
If you have an original Snow Leopard DVD, then things are easy. You start your Mac holding down the C button and follow the guided procedure. It might consider to erase your current hard disk (or main partition) using disk utility.
Once your Mac is ready, you have to install al the latest updates, your Mac should be ready after 2-3 reboots.
Now the tricky part. I have experienced some problem with the “download” button that you find under “purchases” of your App Store.
Clicking on the Mountain Lion link on the left it gives me something I don’t want: “Item not available” in your current store.
Clicking on download, on the right, it shows the button “download” to grey out for couple of second, followed by a spinning loading icon, but then nothing happens. The “download” button re-appears.
How to fix it.
Once you click on “download”, the download itself actually starts, but you don’t see any icon in your dock. I believe this is due to the lack of launchpad for this version.
You restart your Mac and at the next boot the OS X Mountain Lion icon is in your Dock. However is paused.
If you click on resume, nothing happens.
You should be able to resume your download following this procedure:
i. go to your App Store
ii. sign out
iii. restart your Mac
iv. go to App store and sign in again
v. go to Purchases and click on download/resume
At this point your download should start. Check your network status under activity monitor.
You can probably try step i. and v. directly.