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I don't think you can buy "unwanted copies." The game is automatically redeemed to your library, you can't actually add it to your inventory I believe.
He simply wants to buy gifts that someone has on its inventory.
Free bump for my friend!
Ah my bad, that actually makes sense. Although anyone having a copy in their inventory and not redeeming it before today is most likely a trader and won't sell it for just one key. Either that or they didn't like the look of the game and somehow acquired a copy anyways.
buy all unwanted left4 dead 2 copies, since its free at the moment