I am going to make this as easy as it gets!
With 5 easy steps
1) Buy item 15% OVER guide price (1 item)
2) Sell item 15% UNDER guide price (1 item)
3) Subtract the difference
4) Buy items 1gp over minimum price
5) Sell items 1gp under maximum price
34k profit per helmet x 10 (limit) = 340k per trade (8 trades/h = 2720k/h)
Please note this item is hard to get a hold of so don't expect 2.7m off the bat expect something more like 500k with your first item.
I REALLY suggest against using something like this where you have the potential to lose money only try higher priced items as shown when you have a solid grasp of loss.
Note: Aim for 100gp+ Profit/item
Note: Buy/Sell limit does not exist as long as you get rid of your purchases through sales in the GE.
Note: Prices with a price margin of more than 5k normally don't last more than a few hours so try to find them while they are hot! (Barrows tends to have massive gaps quite often)
Note:With lower priced items you are able to get MANY more trades in per hour so the profit is sometimes just as good!