I want to sell a bonus key code of Lego Minifigures

You could create a thread on http://www.steamgifts.com/trades - stating what exactly you are offering and what you're expecting in return (games, ingame items, ...)

seems there's alredy a thread with someone looking to buy what you're planing to offer: http://www.steamgifts.com/trade/MdhiE/

since you're new to trading I suggest you also read through this thread: http://www.steamgifts.com/trade/tIA8K/trading-guide-humble-advice-from-yours-truly

cheers =)