You got scammed by an impersonator most likely. They always target new traders and impersonate high rep traders. Best to check the steam profile of the user that added you with the real profile of the trader they are impersonating.
I bet the scammer offered you something you couldn't resist and tricked you into going first without you actually checking him out properly. :P
You can use Enchaned Steam, Enchaned Steam client or adds-on on browser by just search it on Google Enchaned steam , quick Steamtrades profile from anybody steam profile, and the full Feature you can find on their website, which very helpfull for trading.
I traded with Amidala and disagree with you.
Amidala is a fair trader who responsed quickly and polite.
I do not know what happened in your case or not happened but I recommend him/her as trader.
Sorry it's the impersonator. Didn't come back to this thread to update. I apologise!
Amidala ( http://steamtrades.com/user/amidala ) Scammed My Sr 3 The Full Package And Speedrunners ! Please Be Careful Of This Scammer .