What about sports arbitrage, I can see that working out quite well but I'm not too familiar if that is a common enough occurrence to really happen. In sports arb I dont see why the user can't just bet on both outcomes and simply profit on the same match. I've seen the concept before but I'm not sure why its not being more widespread than what is is currently.
Arbitrage in sportsbooks is quite common. As long as you have exhaustive match results A and B, you can follow this formula for determining whether the arbitrage is +ev or not:
(payoutA - 1) * (payoutB - 1) > 1.
For example, the obvious case is two 2.01x payouts: this guarantees a profit regardless of the result, and the equation returns 1.0201 > 1