You can't shop around Wall street and find arrow-debreu securities with guaranteed positive returns. That's literally offering arbitrage against your own position, which even the retards at WSB don't do. Now show me instances where idiotic market makers offer such rates, that is, set their profit margin to a negative real number. The former is riskless profit as described (bet on both sides), whereas the second isn't. If I fix it at 1.85 and 1.95, then perhaps I'm way off but at least I have a non-null profit margin. If I decide to fix my returns on a coin flip with unknown probabilities at 2.1 for heads and 2.05 for tails, then I am an idiot. And what does that entail? If A and B are the returns for both side, then surely 1/A + 1/B < 1.
