Apple has begun selling redemption codes for OS X 10.7 Lion and OS X 10.8 Mountain Lion on Apple.com. Each code costs US$19.99 and allows the buyer to redeem that code inside the Mac App Store so they can download the OS. At first, the move may seem an odd one since anyone can download OS X 10.9 Mavericks for free, but as TechCrunch notes, the sale of the older OSes compensates for the lack of availability of those versions of OS X on the App Store:
We did some asking around about the thinking behind this particular arrangement and the nut of it is that Apple only offers one version of OS X at a time for purchase on the Mac App Store: the current one. But OS X Mavericks, though it does support some devices as far back as 2007, still has a lot of compatibility gaps for old Macs.
If you've previously purchased either Lion or Mountain Lion, these are freely downloadable from the Mac App Store, but this new arrangement allows customers who may not (for some reason) have owned either one to purchase new copies.
This will also allow users who have old Macs running Leopard or Snow Leopard to upgrade to 'new-er' versions of OS X.
After a buyer purchases a code, they will receive it via email. Buyers can then copy and paste that code into the Redeem section of the Mac App Store and their download will commence.
Apple OS X Mountain Lion
Apple OS X Yosemite