There’s an old scam that runs the same way. On a 2 outcome wager like which team wins a game send 500 people prediction team A wins and 500 people team B wins. For the 500 people who got the right one send 250 team C wins and the other 250 team D wins. By the time you’re down to ~7 people they all received 7 winning predictions in a row, then you ask them for a bet on a ‘sure thing’ for the 8th game.