There is a few things you can do to prevent of being scammed:
If someone ads you where you don't know the steam trade profile off
Ask him to give the link to his steam trading
On team trading you can see a link to his actual steam in browser
Like this:
[You must be registered and logged in to see this link.]Then in the conversation where you are talking to this guy
click on the little arrow with his name and show steam profile
If these 2 profiles match it's the real guy.
If you are not sure if this guy is a scammer you go to this website:
[You must be registered and logged in to see this link.]Paste their steam URL in the search bar and You'll find his profile
[You must be registered and logged in to see this link.]It shows friends and people who have scammers among there friends
and ofcourse if that guy is a scammer or not
Like this one:
[You must be registered and logged in to see this link.]If you are in a conversation trade and the guy is talking weird
And is saying multiple times that he is not a scammer while you did not say anything about that
He could be a scammer while he is still undetected in steamrep
Try to let him give his side of the deal first.
Hope this is enough info for now