Tetsu helms usually have 200-300k margins. They have a limit of 2. So the way you would merch them is by instantly buying one (say, +10 or +15%) and then reselling it 1gp less.
Then, put in a buy offer for 200-300k under that and be patient. Once it buys, sell it at the previous higher price.
Since you know the price range, you can probably repeat this a few times in the next 12-24 hours. Of course, prices change, so if you find that the helm won't buy or sell after a few hours, then you'll need to increase or reduce your price.
I actually tried the Tetsu helm by putting it up more % and then sold it for it's current price and really
HOPED
I would get back more to what I had bought for but no I actually lost money unfortunately which is ok was only a little bit.