Hi,
You could, if you want to, check that the key has already been used (although, as @Jambrix said, it is very likely that the original owner used the key himself to play the game). It only takes a few moments and then you would know for sure… one way or the other.
To check whether the key has already been redeemed, go to https://account.battle.net/overview and type in your key code in the “Redeem a code” box, then click Redeem Code . If the key has already been redeemed… you will get a message similar (or identical) to this one (click on the screenshot to zoom in):
Also, Diablo III has a “Starter Edition” mode… you can install it on your PC, even without a key code, and test whether you like the game and whether your system runs the game without hiccups. The Starter Edition has many restrictions (for example, you cannot go past character level 13 and it stops once you kill the first boss – the Skeleton King)… but it should be enough to sample part of the game.
To install Diablo III on your PC, you need to use the Battle.Net Desktop App, click on “All Games”, scroll down and select Diablo III and then click on Install. (see screenshot below)
(it is best to download the game, since the version on the DVD would be an old version… which would need to be updated before you could play it).
Finally, as you already know, Diablo III is available in several bundles… which can be a bit confusing. I provided some info on those to another player who had a different question, but I thought my answer could also benefit you. You can read it here:
Best of luck with this.