no you are not doing anything wrong ;)
since you hid his offer, i think it's not reportable anymore. It is forbidden to do unnecassary posts like "that hat is duped", but as he also made an offer, i think it's okay. but i lol'd at his proof "original item ıd and your items id are diffrent", people are really thinking like this? so no, i don't think you can report such people, even if they are dumb like hell.

That guy was an idiot, but he was still correct.
Someone calling themselves "Dat Nigga Ova Der" has the other copy[tf2b.com] of your hat[tf2b.com].
You can see that both of your hats have the same original id, meaning that they are effectively the same hat; yet you and him have it in your backpacks at the same time...
When searching these things tf2items' item history[www.tf2items.com] can be a bit more convenient than the outpost's one because it has timestamps.

Now for the rest of the discussion :
- after account hijacking steam support can restore the stolen hats. But they don't delete the hat if the hijacker already sold it to someone else. Result : valve is the biggest hat duper of the game.
- a hat being duped may lower its value a bit but it's very rarely a big deal. (with maybe an exception for really rare stuff (#1 crafted hats))
- I don't see how "that hat that guy is selling is duped" could be considered offtopic in the trade where said hat is being sold.
- If throwing a lowball a the end is all it takes for a post to be considered non-moderable, I can see a bright future for trade derailing.