I'd suggest running a scan, with an anti-virus software, something like Malwarebytes, and Avast usually works, then you should change your Steam password, once you're sure it's safe, then while it's non-compulsary I'd change your Email password from a different PC.

Finally, send in a Steam Support ticket with all the info possible, screenshots of the trade, and a link to the account of the attacker et cetera.

There's a thing called phishing, where you click on a link that downloads a virus, which can steal credit card numbers, and your Steam account.

Let me "phish" up a definative tf2 guide about the subject.
Ah, here it is.

http://steamproxy.net/sharedfiles/filedetails/?id=350320042&searchtext=avoiding+phishing

I'm going to have to give you the most detailed thread of the year award!
With outstanding Grammar, and such an inspiring Spirit, I give you this trophy, it's simply painted Iron, but I know that with your fantastic and Fighting Spirit, this means the world to you!
Thank you, Sir. You have gifted the world, a 2nd wind, and we will use your knowledge together, and we will use it for peace!