From TF2 Wiki
Scammers are people that try to steal your items. There are several way to do this, But ofcourse valve wants to prevent scamming. when trading came out,the trading screen used to have 4 boxes to put items in, so allot of items got scammed back then. but valve updated the screen to 8 boxes, so scamming is reduced. Valve also added a warning system for description and nametagged items, and for trades that contain more than 8 items.
Ways to get scammed: Never trust people saying:
-Trading games for items!, they will ask you to give your items first, and they act nice, add you and built up trust, When you give your items, they will break all contact with you, and they won't give the game..
-Trading 1 item for 9 or more items, because all these items don't fit in 8 boxes, you will need more trades, so he or she wants to have your items first, and promises to give the other items in the second trade.
-Description and nametagged items, For example: They will rename a vintage hat to an unusual hat: Vintage Bonk Helmet -> Unusual Bonk Helmet, The trade screen warns for this saying: This item is a renamed Vintage bonk helmet. Always pay attention.
-Quick Changing, They will have an vintage or unusual or genuine item (better than non vintage), and a normal item. When you are prepared to give allot for this, and both of you are ready to trade (example: you trade 3 non vintage hats for a vintage kabuto), he will change the vintage kabuto to a duplicate, non vintage kabuto. And again, The trade screen will give a warning that he removed the vintage kabuto and added a normal kabuto, so always pay attention and you can prevent getting scammed.
- Gift wrap scam, He will trade you and say he has a gift wrapped item because it's non tradeable, so he wants items from you first, and then he would give the giftwrapped item to you, but he won't. Since a certain patch ,you can now put giftwrapped items in the trade boxes, so if someone offers this to you, ask him to do that.
Note that some people actually try to do this fair, and some people will for example give the giftwrap afterwards, some people will send a game afterwards, and some people will trade you again after recieving items from you for nothing. so don't blame people if it isnt obvious they will scam.
How to Prevent scamming?
-Keep trading between the 8 boxes of the trade screen. -never trade for games exept it's a good friend you trust -report scammers on their steam profile
I have been scammed! what can i do?
-Report the scammer and keep his steam information ( ID,community profile,etc) -Create a steam support ticket if you dont have one and Contact steam support -> [HERE] <- -Read The trading guidelines : -> [HERE] <-