On both the Mac App Store and GOG you can just buy a game and play it without the need to first prove you actually bought a genuine version of it and then doing the same check again every once in a while. The game is installed, so you can just play it without an internet connection, make a copy of it on an external hard-drive or whatever.
This is only true for MAS apps that don't do receipt validation. Which, admittedly, is a lot of them. I've never actually seen a base app that is doing receipt validation, it is more common for iAP. Apps that have receipt validation should require sign-in on first launch with the associated apple ID if you copy to a new machine (or restored machine).
GOG, currently, lets you grab offline installers, though they keep burying them further and further in your account console.