Say you're running Snow Leopard, and there has been a 10.6-compatible version of an app, but the newest version requires Lion or greater. Or, say an app recently received a UI overhaul, and you'd like to buy the version that came before it was changed (perhaps the folder structure changed and broke your applescripts, or you just liked the old one better).
App Store only shows the most recent version of the app, so is there a way to get it to show an older version? I know, you can sometimes try buying directly from the developer, but that's not always an option, and it'll become even more rare as time goes by.