Q u o t e: Hello
Today i turned on my computer and tried to log in Diablo 2 and it said that my LoD key is in use by *some random dude*. How is this possible? And does it really matter where you buy the key since it's valid when i enter it on battle.net and you get the new 26 digit key anyway...?
If you bought the key from a 3rd party site and not Blizzard, then it is a problem. These sites are not authorized to sell them, and they will often sell the same key to multiple people.
It doesn't matter if you register it on the battle.net site or not, the battle.net site does not verify keys or anything like that, it'll take pretty much any key, it's just used for convenience when you want to digitally install the game for example. The check for a key occurs when you try to connect online.
Your best course of action is to attempt to get a refund from the seller, and purchase an authorized key from the BNet site or a retail store. Blizzard can't intervene in this case.