Is there a way to purchase incompatible app from Mac App Store
Unfortunately, the only way I've found the App Store to cooperate with that was to create a user account on a compatible system from which to log into the App Store and download the now-compatible app. For example, on my Lion system with a 32-bit EFI, I was unable to download Mountain Lion or newer to install on my daughter's MBA. I had to log into her Air in my account to download the OS upgrade for her.

Not convenient.