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...the next pick to the people who saw you pick the "winner". Now half of those people see one team, the other half see you pick the other team, and whoever saw you pick the winner thinks you've got a 100% accuracy rate over two games. You could do that for a while and then offer to sell your pick for the Superbowl. Starting with a big enough group in the beginning, this might be really lucrative.
But is it legal?
Pretty easy if you keep narrowing your email pool when people see you pick a loser.
And how would you know that’s happening?
....When you get investigated for fraud? Likely when they check your outgoing. But also by communicating with other targets of your fraudulent service. I doubt you will send 80 million emails manually, but go right ahead and test that.
How would you know fraud is happening to start a lawsuit…?
You seem to be putting facts together after the fact.
...what do you think is going on here, in this thread?
It's talking about taking peoples' money based on your (fraudulent) ability to predict the outcome. There will be victims in the form of the people whose money was taken. Some of those people will see that the result didn't match. The fraud will be evident to the defrauded victims.
The original question was whether it was legal, not whether they could get away with it. If they did get caught, there is a very high likelihood they could be convicted of fraud.