There are inefficiencies that make certain bets positive EV if you are smart enough. It's usually a combination of playing in a weird way and having some insight that the maker of the game (the oddsmaker) didn't see. Gambling establishments don't mind because there are few enough of these and they will ban you if you take too much money from them.

Winning sports betting players often go on to set odds.

Sportsbooks make money by taking bets on both sides of a game and offering odds that work in their favor. For example, even on an "even money" bet, you might have to bet $105 to win $100. The more one-sided a game seems, the bigger the gap between the odds on either side because the sportsbook is trying to manage its risk. As people place bets, they adjust the odds to balance the action. The sportsbook isn't banking on you being wrong—they want enough bets on both sides so they win no matter what. The difference between the odds is basically their "fee."

As a professional bettor, you're not really outsmarting the sportsbook—you’re trying to outsmart the public. The key is finding moments where the crowd is wrong enough that betting the other side makes sense, even with the sportsbook’s fees. That means you’ll often skip betting when the odds are pretty accurate.

Most sportsbooks will limit how much you can bet if you're too successful, but they usually won’t ban you outright.

Many sportsbooks actually do not run that way. The name "sportsbook" implies that they do, but that is an older style of betting that has fallen out of favor. Modern sports books usually use fixed odds set by an oddsmaker (in modern times, algorithms set by the oddsmaker), but those odds are allowed to float with the probability of the outcome changing. I believe they take supply and demand into account, but you actually are betting against the house. That prevents the kind of trading against the crowd that would be normally viable.

The Hong Kong horse race track was a famous example of market-priced bets where the book was run the way you said and the crowd was exploitable in the way you are suggesting. It was one of the last books to work that way.

I can see the misunderstanding, but I was not actually describing parimutuel betting (horseracing). In parimutuel betting the odds continue to change up until the race even once you've placed your bet ensuring that the payout is always the total amount wagered less a fee kept by the house.

Sportsbooks will open lines intelligently, but they absolutely do move the line in response to market forces in an attempt to balance money on both sides, because when the money is balanced, they are guaranteed profit.

It's true that when you make a sports wager, the house is paying you out of their wallet. It's also true that they employ a lot of energy and expertise in order to open the betting at accurate odds. However, no corporate, end user facing sportsbook is themselves fading action on one side of the match intentionally. They aggressively try to balance money on both sides so they can guarantee a profit.

>It's usually a combination of playing in a weird way and having some insight that the maker of the game (the oddsmaker) didn't see

Apparently exactly this. The people that I knew where always discussing the fitness of certain players and how that'd impact the game and stuff like that. Though it could've also been that they were on a long long lucky streak, because they minimized the risk with such considerations. At least t hey were not ruining their own lives

I have a friend who professionally plays video poker, and has been doing that for a very long time. He runs Monte Carlo simulations to find his strategies around various kinds of promotions and specials that casinos offer. He has about a 2-5% edge whenever he plays, and maximizes his bet size and machine time to take advantage of this. Casinos don't care about this sort of thing because the strategy he plays is usually batshit insane compared to how you would think video poker ought to be played (eg "throw away cards from a flush to mine for a straight flush" is a frequent rule he uses), and is very complicated. They lose ~$10k a week to the three people like him who can do this, but more than make up for it in the rubes that come in the door from those promotions.

These sorts of inefficiencies, and often even true arbitrage bets, show up in sports betting because the bets you need to make are so complicated. There is a team at Susquehanna that does sports gambling as their form of trading, and they will sometimes play these sorts of arbitrages against bookies. I remember hearing about a perfectly-hedged arbitrage of 8 different bets from one member of that team in a specific gambling forum, but the bets were all so arcane that very few other players were playing each one.