Not really. It can have two winners. I bought at 300$ and sell for 600$, because I want to buy a camera. Then the prices rises to 900$, like you predicted. So buyer and seller win.
Rather: the guy you bought from (if he got for free) won 300$, you won 300$, the chap you sold to won 300$, ahd the chap he sold to lost 900$.

No, the last guy didn't lose $900, he got bitcoin...