Sadly you can't too much beyond reporting them. Valve has this weird policy about trading, so that when you accept a trade, they always say it's your fault you got scammed. They will most likely ban the guy and move on.
You gotta be always weary of scammers. They usually give you links to their sites, ask to check if a stat of the item is "in order" or it's no duped then ask you to trade it to someone. If the price is too good to be true, always get suspicious. Real traders shark very hard and will offer you half or 3/4 of item's worth since trading bots are a thing. That's why sca