Arbitrage sports betting is legal, though it's frowned upon by bookmakers.
Arbitrage in sports betting is when someone places bets on all possible outcomes of an event at odds that guarantee profit, no matter what happens in the event.
An arbitrage bet is usually possible when there is a discrepancy in odds which would allow for a profit to be made by covering all outcomes. This is done when bookmakers disagree on odds or when there's a mistake in the market.
For example, betting site A has the Indiana Pacers as a -270 favorite to beat the Detroit Pistons, who are a +350 underdog. At site B, the Pacers are at -370 to win and the Pistons are +450. An arbitrage bet would go as follows:
$80 on Indiana -270 for a payout of roughly $110
$20 on Detroit +450 for a payout of $110
The individual is betting $100 total and no matter what happens, the payout will be $10. Of course, the bettor can up the bets for a bigger payout and if you want to turn a larger positive, it'll take time.
Since this doesn't involve knowing anything about sports, it's more of a mathematical approach and is frowned upon by bookmakers. While it's legal and it will lead to positive winnings, it's also a process and one that could eventually get you banned from a sportsbook if you're caught.
While it looks easy based on the example above, numbers like that aren't easy to find. In order to turn a positive, you have to scope out numerous sportsbooks and find the correct lines so you can make a bet on both sides.