[Discussion] Just got scammed, how do I report the guy and possibly get my items back?

I traded with someone for a Five-Seven CH FT Blue Gem and he switched it out with his friend after I inspected and gave me a solid gold one instead.

I then asked him to give the keys back, or I would report him, and I'm guessing he got spooked by that because he then blocked me.

Where would I report him to get him banned?