I suggest you use enhancedsteam if you use either Chrome or Firefox, trust me.
first time i heard of that it actually looks interesting may be a good plugin for new comers, i personally have never been scammed but i know what to look for but i am getting annoyed with how many people keep adding me trying to scam me i actually did a epic 1 the other day and while i kept a scammer talking i added the real trader to friends and invited him to the chat was funny :D
This is not always the case you could always ask them to send the key code first as i have had people trade with me and they had no rep either some times it is too good to be true you will have to judge them yourself as i got a batman origins key with the DLC of a guy and tried the key on my account first and it worked so i traded him Bioshock infinite gift after which was a great deal. =D
to avoid being scammed follow some simple steps which are:
once you have said trader on your friends list ask for their steamtrades.com link
now view their reps if they have very low rep do not accept serials keys etc only straight trades via steam trades
click view steam page if they are on your friends list and it says add friend then it means you are talking to a impersonator
final the breakdown
reps stand for reputation which states if they are trusted or not
impersonator stands for somebody who has made a profile that looks like a high reputation trader to gain trust
lastly i would always advise against trading outside of steam games but if you really have to follow these steps and also check their steamrep.com and it should go smoothly.
wish you all luck happy trading guys