Arbing is not Forever
Arbitrage is a system itself and it is one which is not easy to make sustained profits online.
Bookies are not daft. They have beancounters actively looking out for arbers. In some instances you only have to have one winning bet to be classed as an arber with the result that your future bets will be severely restricted.
Forget online bets with bookies if you want to be a winner. Their programs are finely tuned to stop regular winners from winning whilst they quite generously accommodate mug punters.
Arbing on the exchanges is possible but not easy to get matched at the right time - there are far too many bots milliseconds in front of you.
The best way to make money is to find flaws in the odds themselves. If a bookie or exchange layer is offering 20/1 about an item you think has a 10/1 chance of winning and if your method is sound you can not fail to return a profit.
You can only do that online at an exchange or with cash bets though as those damn beancounters will start to restrict you if you win regularly.