In college I did extensive research on this amongst Bodog, 5dimes, and another third party vendor considered 'reliable'. I'd scrape sites hourly because lines move like any market will, and I did this for all 17 weeks. I don't think there was one instance where there was an arbitrage opportunity (my algorithm would go 4 game-pairs deep, so it wasn't an exhaustive analysis but rigorous enough). Even if theres large disparity between bookies, the fact that the industry accepts -115/100 as fair would require a huge mispricing by a bookie for you to arbitrage successfully.