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You can't trade games that you have already activated in your library. They must be in your inventory to be tradable/giftable.
When you buy a game from the steam store you choose the option to store it in your inventory instead of activating it in your account.
OR you use sites like Humble Bundle to buy multiple games for a very good price and then you trade the codes to other people on steamgifts. That's what 99% of the users here do.
Chivalry