Assuming you know sports betting, you are probably familiar with the many different sportsbooks that are around, some way more established than others. You may think "These other sportsbooks are useless," but if I came back and told you that you should want these inefficient sportsbooks to be around because you can make risk-free profit from them, you shouldn't think I'm crazy because many people are making 6-figures a year doing this lucrative task.
When you sports bet, you HAVE to know the sharp (i.e. liquid) sports books. As far as the lines that you could trust, they include Betfair exchange, Pinnacle, etc. Keep note, however, that these sportsbooks are European, meaning that they CANNOT be used for betting in the US. They take lots of volume, have very tight lines, and are extremely efficient.
Going on a little tangent, you can think of sports-- you want to have the probabilities in your favor. Yes, there is a chance that you can lose money, and yes, there is a chance that you get limited. However, the probabilities are ultimately what matter. If I get $6 every time I flip a coin and it lands on heads, but I have to give you $5 every time it lands on tails, I will still take the deal no matter what because in the long run I will be a millionaire if the coin is flipped millions of times. The same concept applies to sports betting, only with a ton of subtleties.
Back to the topic. If Pinnacle and other sharp books all make a game +130/-140, then one should be pretty thrilled to buy the favored team for -120 on BetMGM. To give reference,opportunities like these exist. Tangential to the coin flip example, if I found arbitrage opportunities like the one articulated above, and I extrapolate the probabilities a thousand times (or even 100), the chances are extremely in my favor that I make money. Inefficiencies are all over the place in sports betting -- It's about taking advantage of these opportunities and making the markets more efficient. Cryptocurrency in 2017 was not efficient, and many people made massive amount of capital through arbitrage. The same thing applies to sports betting right now. These opportunities don't last forever -- Take advantage of them while they are still around.
Another imperative concept to understand: There is a lot of merit in the relationship between implied probability and sportsbook odds. For example, let's say the Spurs and Lakers play each other, where the Lakers are favored -150/+190. To calculate implied probability, you do (150/250 +190/290)/2 = x. The answer gives you the implied probability of the Lakers winning the game (since they are the favorite). Then, as you would expect, if you want to calculate the probability that the Spurs win, simply do (1- x). The beauty of sharp betting sites is that you could price "odds boosts." Let me give you an example that is a little more complicated. I see a boost on DraftKings where the " Rockets and 76ers" are +290 for both to win. I would initially calculate the probability that the Rockets win, then the probability the 76ers (ideally from a sharp sports book like Pinnacle) and then multiply these probabilities. This would consequently give us the implied odds. This is the part where you look at other inefficient sportsbooks to compare the odds. If I see a sportsbook that has a parlay with the Rockets and 76ers at +300, I shouldn't take the bet. However, if I see a parlay that is +250, I should want to take the bet any day of the week (See how this works?) Calculating the expected value () will allow us to further determine whether the bet is good (I will leave that to another time).
As far as good resources to utilize, sportsbookreview is the perfect tool. You can think of it as a central database for sports betting, with countless different leagues, the sharp sportsbook that offer these competitions, calculators, etc. (I have no affiliation with this site, just simply a tool that is very helpful for betters). Also keep note that the site is totally free to use. It shows real time odds from sharp sportsbooks that you can follow. Although there are many other resources that I use at my disposal, sportsbookreview is by far the #1 because of its great UX, big and accurate database, etc. (You get the point).
Currently, I use an arbitrage bot that shows me real time opportunities like the ones I mentioned above to give me an edge on the market (Now, the sportsbooks aren't making money off of me haha). However, just like letting your ego win in traditional markets, crypto markets, etc. (If you're wondering why I use crypto as an analogy, it's because I also trade and do similar quantitative stuff in that market), you will lose in the long run. In other words, you HAVE to be cognizant of the penalties and limits that you may get if you place huge bets on arbs with small upside. BE SURE to mitigate massive bets as they are a great way to get your account limited.
Remember that arbing is turning a ton of profitable and unprofitable bets to little profit. You always have to manage your risk in this space, just as how you would manage your risk in trading options, stocks, you name it. Position sizing, risk management, and all the fundamental concepts that go into investing are applicable to sports betting, making it easier to grasp and get the hang of if a trader is entering the space. Keep note, however, that you do not need to be a trader -- just understand the fundamental ideas that all sports betters are aware of (most, at least). Also important to articulate, DO NOT watch the games you bet on. It is very time consuming, and most critically, tough on your mental health. Just make good bets, put the probabilities in your favor, and win!
To put a close on this write-up, you should join a community of smart bettors. Exchange ideas with each other, ask questions, contribute, and most importantly, learn how to make these markets more efficient. There is a big gap right now in sportsbooks in the US, and anybody like yourself can take advantage of it if they know what to do. Yes, this may be overwhelming for those that are new to this space, but I promise you it is well worth it in the long run if one puts in the time to learn and get better. Coming from a trader in many markets, I have found sports betting to be very similar -- There are a ton of fun attributes about it, but you are also dealing with your own money, so be sure to practice proper risk management, allocate time to the right resources, and discuss with people that have been in the space for longer than you have.
Let me know if you have any questions. I would love to address them or share my experiences in this space. Let's all make money together!