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.
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.
Like the previous versions of Diablo, Blizzard will be releasing Diablo 3 on for both the PC an Mac platforms. The game is being planned for a simultaneous release on both Windows and Mac OS X platforms. Blizzard has also announced it has "no plans" to make the game available on other platforms - so the X-box games will have to break down and go to a regular computer if they want to play it...
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.
Hard Copy - 1989 Angels was released on: USA: 3 June 1994
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.
Hard Copy - 1989 Horse Whisperer was released on: USA: 3 August 1998
diablo 1 is about 100mb diablo about 800mb and diablo 2 expansion another about 800mb