I'm running Mac OS X Leopard 10.5.8 and need to install an app that can only be downloaded from the App store. If I download it to a newer Mac can I install it on my main Mac running 10.5.8? When I click "Buy App" it starts the install immediately. I would like for it to just be a download that I could transfer to my other Mac and delete from the mac downloading the App. Is this possible? How?
An install on Mac OS is generally just copying the relevant files into place. Usually, you'll be able to copy an app's bundle over from one machine to the other. The only thing is that the App Store will have some sort of DRM, so just copying it may not work. I suspect that the answer is no, because you'll need to sign in to the App Store, but I haven't tried it. (On the other hand, I've installed Lion on a Leopard machine via a disk without Sow Leopard.) Additionally, Apps that are downloaded from the Mac App store will carry minimum system requirements that might prevent them from running altogether, even if you theoretically circumvented the DRM from the App Store.
No, you cannot. The DRM associated with App Store downloads requires that you log into the App Store application. the App Store application will not run on systems below 10.6.6.
There are very rare exceptions to this rule... applications that do not install from the App Store, but instead download an installer program. Lion itself and Xcode are the only two I can think of that behave this way.
Some apps on the Mac App Store do not check if an app is registered at launch, so it is possible just to copy the .app file to an older Mac. However, an app that is exclusively available in the Mac App Store is most likely not compatible with Leopard. So, even if the developer does not enforce the App Store's built-in DRM system, it is not likely that the app would run on OS X Leopard.
When you click "Buy App", it should start downloading, not installing. Make sure this is the case.
In any case, once the app is downloaded, you can follow any of these instructions to burn the install image to a CD or DVD. You can use that to install Lion on your older system.