> What made one gambling and the other not? What makes any given bet a legitimate financial trade versus pure gambling?

Legally, it's whether the bet meets the legal definition of gambling as per the legislation applicable in your jurisdiction and any binding case law. There's no universal answer.

But from an ordinary language perspective, the difference is whether it involves an element of skill sufficient (at least in theory) to win in the long run if you possess that skill. For example:

Roulette - generally gambling

Blackjack - usually gambling these days, sometimes not if count counting is feasible

Poker - not gambling

Stock market investing - not gambling