Because the physical copy comes with a CD key, and that CD key creates your account, few people will accept a used copy of Diablo 3 because it is worthless without an unused CD key. You could possibly return a sealed copy, depends on the policies of the place you are returning it to.
No I think it is for PC
You can purchase Diablo 3 for Mac computers online at the Diablo website; the hard copy version can also be purchased at most computer game stores.
Yes, Diablo is in Diablo 3.
No it can't if you want to play with your friends online i recommend either both getting a disk copy or both getting a download for it.
No. There are Diablo 3 teaser videos on the Blizzard website.
Mephisto is mentioned, but does not appear 'in person' during Diablo 3.
Diablo 3 is out now, it was released on the 15th of May 2012.
No, there is no pirate class in Diablo 3. If you mean to pirate the game, you cannot do that either.
diablo 1 is about 100mb diablo about 800mb and diablo 2 expansion another about 800mb
It's been over 13 years since Diablo 2 was a true Hype, so this would give you a perfect reason not to worry if Diablo 3 would be your 1st Diablo game experience.
Diablo 3 was released on May 15, 2012.
The requirements can be found at the official Diablo III website.