None? Thing is: Once you've opened it, people can't be sure whether you have installed it or not.
And since the first install ties the game to your Steam account forever, it means that after the first install, it's impossible to return/resell it - since it's yours for life (or until Steam goes bankrupt or something).
Note that, as long as you hang on to your Steam account, you can re-download the game as often as you want.
Cheers, LT.