I saw it advertised in trade chat and whispered the guy. On my realm, those advertisements are very rare and we basically don't have bot spam, so I just gave it a chance. Before trading the gold, I whispered him something like "Ok, so you sell me an Argus Heroic kill for 200k, right?" and he replied "Yes, I sell you an Argus Heroic kill for 200k". I'm pretty sure that you can get your gold back that way if you get scammed since a GM can easily see the terms of the trade in the chat log.
I play on a german server and he only spoke okish english and no german. I wouldn't have done it if it had been a larger sum.