Link the profile. Report to steam and steamrep (you will need some screens of the convo as proof.)
hit the print scrn button on your keyboard (if you still have the chat window open.) regardless, linking the person profile to this claim would do you good also. people pretend to be other people from these forums pretty often. A private steam profile is one give away. Having multiple user names that have nothing in common at all is another. The person not having a thread or posting in your thread, on these forums is yet again another give away.
watch out for http://steamcommunity.com/id/_vacsek and http://steamcommunity.com/profiles/76561198077997006 also. They impersonate people from here, who have very good rep.
He just scammed me out of a Farcry 3 key. Somehow his steamrep is clean. I should have checked his rep here.
After 5-6 reports, he finally got tagged as scammer. Steamrep admins move very slow unfortunately. When I got scammed, there were already 2 reports under his name, but he wasn't marked as scammer yet. Oh well. Lesson learned: profile comments are never to be trusted!
he just took my dues x cd key without paying me