That's the interesting theoretical bit. The reality bit is where you have to ensure that you can get both bets checked in with the bookie(s) involved at the pr

That's the interesting theoretical bit. The reality bit is where you have to ensure that you can get both bets checked in with the bookie(s) involved at the price you wanted - and arbitrage opportunities tend to show up more when the market is moving fast. Further, just like the stock market, arbitrage acts to keep the market stable- this is why most bookies have more or less the same odds on the same games at the same price. One also must factor in the transactional costs in moving money around depending on who you are using for bookies, and also realize there are already lots of guys out there running software 24/7 looking for arbitrage across the entire bookmaker landscape globally - so such opportunities tend to not last for long. The more "fun" opportunity is the middle - when a line moves so much during the time leading up to a game that a guy can make a bet with the same bookie that he wins on no matter what, or at least can't lose on (at worst it's a push - no loss). To find those you have to bet early on what are likely to be fast moving lines, and then be quick enough to get your bets in (and have them accepted)