The problem with PC game keys is (as DarkHououmon already stated) that there is only every one key per copy of a game (nowadays). By registering someone else's key, their key then becomes invalid and won't work again. There are keygens out there that can be used or failing that, you can always contact Bungie directly and tell them you've lost your key and they could give you a new one (for a fee no doubt). Similarly, like LBTLover said, because you've already forked out for the game, running it in emulation or downloaded (or pirated if you like) isn't technically illegal.
Shops are very arsey when it comes to returning PC games and for good cause really. They have no evidence to disprove that you haven't copied the game, cracked it or installed the game and asking for a full refund whilst still being able to access the game. Potentially lying to them to get the game for free. Once a PC game is sold and the key goes with it, the key (from a retail point of view) is useless. If you give the game back having used the key, (again a factor they cannot disprove on the spot) that's a product they can no longer sell.
Your best bet metadude is to get in touch with Bungie. There must be an online help page or a way to retrieve lost keys. It will probably cost you but it'll be a darn sight safer than messing around with dodgy keygens with the risk of landing a virus. It's all up to you at the end of the day mate. This is just my 2p's worth
