I just mean that on Steam - say - you wouldn't be allowed to do this, would you?
Or is it just spare games, like if you have codes for more than one copy?
I am new to all this :D
Here you can trade for gifts that are in your Steam Inventory and for unused
codes to redeem games on GOG, Steam, Uplay, Origin, Humble Bundle, Desura, etc... Usually gifts in the Steam Inventory are more valuable, because you can use the trade system from there which is safer than trading for codes via chat or PM that can't be verified if they were used or not, and the gifts don't expire if they are kept in your Inventory. Just read the first post
to understand better, and if you have more questions, ask away ;)
By the way, bundle games has less value than non-bundle games, unless you want to trade many bundle games for 1 non-bundle game.
Just remember, games in your Steam Library can't be traded (they're already activated in your account), only the games in your Steam Inventory, and keys to redeem games on different services can be traded.