Tomorrow, you sell it to a third person for $10.

Who lost money here?

Really no one I think.

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

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.

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.