It's easy to spot such scammer but I guess not everyone is suspicious enough. As always, if it sounds too good to be true, it most likely is.
My guess is the scam possibly goes like this:
1. Scammer advises victim to place a bet on X, where X is actually picked at random, for FREE. They claim they have insider information.
2. If X bet loses, the scam is over.
3. If X bet wins, the victim goes back to thank the scammer and requests next bet advice. Scammer first asks a cash "tip" since victim won thanks to them.
4. Go back to step 1.
Scammer essentially makes money by advising random bets.