What if there is one iron on the main map. Can I sell the one iron and then buy one from the demand supply to sell also on the same turn?
Yes, you can absolutely sell 1 iron, buy 1 iron, then sell the just bought iron during the same turn. This would be considered 1 'sell' action.
For example, let's say that there is 1 iron on the map and the demand display is empty. For your 'sell' action, you want to sell a total of 4 iron to 2 Ships. The first iron must come from the map. The additional 3 iron comes from the empty demand display. The total cost of the action would be $9.