Value is subjective and essentially exists all in the mind. It's PSYCHOLOGICAL. If humans believe something to be useful, they create wealth through that perception. It's not a zero sum game and even Kenynesian economics wouldn't claim that the economy works this way.
Well, then, why do people worry so much about the economy? We can just all change our perception and say that the dollar is worth a thousand euros, and problem is solved. And and if people who own basebal cards agree to refuse to sell them for less than a quintillion dollars then they can buy the world with one card.
Yeah, if you could do it, you would be a rich man. But humans aren't stupid enough to fall for such a trick. While value is purely psychological, that value assessment is still based on real world utility. Not to say that it's impossible to trick people into mistaking the utility of an object, because some people have that amount of skill, but it's no mean feat.