Excellent question.

Really no one I think.

I'm definitely wrong about something though. I need to think about it some more to figure out what.

A new interpretation came to overnight, as if in a dream. I don't know what's correct, but here it goes.

You and the third person both lost money in some unknown combination. Either you're selling for a lower price, or 3rd is buying for a higher price, or probably both.

In the metaphor, this is aggregated across bazillions of transactions, and ignores benefits like supply and demand are generally evened out over time.

I think the question is flawed... Or at least irrelevant.

Since we are talking about the stock market (and stock brokers don’t make anything of value) a better question would be:

You hire a worker for $2 to make something you sell for $5. Someone buys it for $5 and sells it to a forth person for $10. Who lost money?

Now to make this simplification more accurate, there is a system in place for the $5 to $10 transaction called The stock market where there are bazillions of these transaction. Another question arises. Who is selling something for $5 when they could sell it straight to the forth person for %10? Who has is $10 and why are they paying $10 for it? How did they get that $10? Did they make something worth $10 and sold it for $10?

I think the real answer for who is loosing money is: Workers across the globe are.

Without the middleman it seems possible that some of the buyers and sellers never might have found each other. I don't know.