http://steamcommunity.com/id/costelushh/
this guy will offer you 10$ steam wallet for gmod ... you can add him and talk to him he is a scammer.
he will force you to go 1st.
he is unknown about this, untill he sees this :P
It's actually not easy to acquire a lot of clean rep on ST. People who have it value it and won't squander it on a scam. Steam profile rep however is meaningless since it's easy to fake and bad rep can be deleted by the account holder.
What you're proposing with keys is idiotic. Someone has to go first or else no trades will happen at all. Use your head, decide which risks are worth taking and which are not, but don't go running around like Chicken Little screaming the sky is falling because you ran into one asshole and didn't take reasonable precautions.
You are added by someone randomly and asked for your game in exchange for a even/better game key / paypal / wallet code (new).
The person will ask you to go 1st in exchange for their uber offer.
NEVER DEAL WITH SUCH PERSON even if they have 100 comments on their profile or steamtrade or steamrep etc.
Its not difficult to acquire those comments and rep for any1.
Only trade if you are trading through steam trading facility ... as well as for keys avoid trading them or never go 1st.
The person with key should go first if you cannot arrange a middleman.
and above all dont be a greedy fool.
If a scammer scams you its your fault not his.