I'm not disagreeing with you. If you controlled your supply of precious space material z, yes you could control the price of "space material z."

But if you pull space iron and try to control the supply, you're going to have a hard sell of your space iron. It's not special or precious. There might be the odd ball billionaire that might want to make a stupid "truck" from "space iron," but that's your only market.

If you're a cinema, you say no outside food or drink. Sure some people buy the $15 popcorn, but pretty much everyone sneaks in their own candies and drink.

Long story short - fettered markets are an exception to supply and demand.