This post is in my wheelhouse: I’m a CPA (albeit one who does not work in the tax field) and sports bettor. Few things:
1. Others have said it but I will reiterate in the OP’s example, you’d have to report $110 as other income and if you itemize, you could report the $100 loss and have a net impact of $10 to your taxable income. If you cannot itemize, it would be a $110 impact to taxable income. Make sure you where you are going to file before you start to arbitrage.
2. For those curious about arbitrage and saying they have never seen these opportunities, trust me they are out there. You can find arbs every single day if you have accounts at multiple books. They are generally on prop bets and other more obscure wagering lines, it is uncommon to find them on traditional money line bets. With that said, books are becoming quicker to ban people who are consistently making large dollar wagers on these obscure bets. I know several friends who successfully did this for a while but have since been banned or limited down to a dollar amount where they can’t really do much. Have to be careful or else you won’t get to bet on anything let alone the arbs you may be trying to hit.
3. I saw some reference arbing boosts and promos at books and math says generally you are losing value by doing that (assuming it’s a good boost since they regularly post duds that are actually just regular odds). Sure, you guarantee a profit, but if you look at the expected value (probably of event occurring * odds), over time you would make more just betting the boost straight up and letting it go.
4. If you are curious about whether or not you may or may not have found value in a wager, I’d recommend using Pinnacle as a resource. That is a foreign book that takes a lot more action than the US books and because of this, has more accurate odds. This is helpful when figuring out if the aforementioned boosts are actually worth playing or not.
Best of luck to my fellow gamblers out there. Hope you profited from the mispriced ISU lines last night like I did.