Ignore pressure and do not rush the trade
A common tactic by scammers is to force you to trade quickly so they can change items/gifts in the trade without you noticing.Ignore the pressure to trust the other user. If you are trading with a user who insists that you trust them, they are probably attempting to scam you.
Note that +Rep comments can be generated easily by malicious groups.
Mouse over every item to ensure that the item/gift's properties are correct
Information about the item/gift will be stated here including the quality, name, description and any effects.
Pay attention to the trade log while making the trade
All changes, additions, removals and actions will be recorded in this box. You may also use it to communicate with the trader.
Do not trade items outside of the trade window
If another user requests that you do, they will likely scam you. Always insist to trade within the trade window in Steam. Wallet credit and money cannot be traded or added to the trade window.
Ensure that you are trading with the correct user
Scammers may try to impersonate your friends and other trusted traders. It is your responsibility to know who you are trading with.
Also ensure that you trade using official valve trading servers. I would personally not take this trade up but if you want to you can always cancel at any time and within 2 days of trading.(someone correct me if I am wrong)
Taken from this steam support page.