... But Diablo II's by Blizzard, not Steam.
Blizzard's a bit more flexible. If you send them the jewel case with the CD key sticker, they'll deactivate the key in use by that other person and send you a new one. (Source.) So Blizzard doesn't really care if your copy is used, as long as you actually have the original media and CD-Key. You'll have to wait a month for your new key to arrive.
Steam, however, will not care if the game is used or not. If you bought the game new from a retail store, and have a receipt to prove it, you will get a new CD key. If you didn't, and you bought it used, you have to pay ten bucks, and wait six to eight weeks for your new key.
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Sorry for the off-topicness. >_>'