PLS NO SCAMMERS OR I WILL REPORT YOU TO ALL OF YOUR FRIENDS.
The common scams i have encountered on trading:
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1} LOADING SCREEN SCAM: Guy offers a bundle,says fast trade ,then in a sudden he replaces the bundle with a loading screen, if we doesnt notice, then our items are traded for a loading screen.

2}STEAM WALLET CREDIT SCAM: Guy offers steam wallet money for trade (in my case a pinoy girl) ,Types a long convincing message like its a formal steam wallet credit exchange by steam itself saying "your account will be credited XX amount of money in exchange of the items" This scam is only done by steam offer other wise traders will easily notice the scam,
THERE IS NO WAY TO TRADE STEAM CREDITS.

3} UNUSUAL COURIER SCAM: Guy purchases a cheap cursed courier. Cursed couriers' thumbnails/ picture has a violet highlet similiar to very expensive Unusual couriers. Then renames the courier to "Unusual xxxx"
If the trader is ignorant his items are lost. When i said "nice scam" to that scammer which i prevented from occuring, he said he actually scammed many traders with that scam. So really its possible to scam with this trick
SO READ EVERY THING BEFORE TRADING

4} Trade offer which says that "your account has been found out to be duplicate..blah blah blah, Any thing that smell suspicious is really scams
SO DONT ACCEPT ANY SUSPICIOUS STEAM OFFERS

I HOPE YOU GUYS LEARN SOMETHING FROM THIS PIECE OF INFORMATION AND GIVE A +REP FOR ENLIGHTENING YOU, THANKS.