Very true. Most think it like a casino where money just rolls in.
By the way, to expand upon your analogy, if you are a merchant, you best know what stuff to buy and what the market is doing. Failure to know your customer means you will buy a lot of crap that people do not want.
It doesnt fit ideally, but the old adage holds:
In markets, you make money when you buy; you get paid when you sell.
Seconded.
That's one of the reasons most businesses fails. If a business owner cannot follow up with market trends, that business can only last for a season.