Earlier quoted context omitted.
This proposition, discussed and shown by Piketty in his 2013 book, was not at all obvious before that and even today many people would argue (wrongly) that it isn't true. So if it's a natural law of wealth, it is a very recently proven one. https://en.wikipedia.org/wiki/Capital_in_the_Twenty-First_Ce...
Marx presented this idea back in the 1860s. He phrased it a little differently and built the idea up from the idea of exchanging commodities. In Marx's text, he discusses the history of commodity exchange, how people usually exchanged Commodities for Money which they exchanged back into other Commodities they needed. (E.g. I'll sell you my wool for coin, which I can use to buy food.) He called this CMC exchange. (Com…
I'm guessing this is oversimplified, but I haven't read the book. Otherwise, the simple fact is no trade in business is guaranteed, there will be plenty of such exchanges that lose money due to speculation. At the very least your buyer could pull out or go bankrupt before finalising the sale resulting in a fire sale and a loss.
Of course, by sheer probability if not talent, some people will win more than they lose and get ahead. The real danger is the plays that an excessive force of money can enable, like predatory and monopolistic practices in buying up a market (or legislators), or driving a competitor out of business. That's where money really makes money.