I'd be meaning to reply to this but I was "otherwise engaged" when you posted.
I appreciate you've realised it was too good to be true but here's a brief guide to cashing in when backs at the bookies are bigger than the lays at the exchange AKA arbitrage betting for future reference.
There is money to be made in this situation as you've realised as bookies sometimes get their prices wrong or the exchanges move before they can react.
Before taking a price think:
1- are the markets you're comparing definitely the same? This is especially true in handicap markets where minor differences could lead to you losing out big time.
2- has the bookie made an error? Check the basics like have they got the odds for the home and away team the wrong way round or is there a typo in the odds? You might think you can take advantage in this situation but bookies will void your bets on the basis that they made a palpable error or "palp". Sounds unfair (it is, I suppose) but you have to be aware of it because it does happen.
3- are the rules on both markets the same? This is especially true for tennis betting where some bookies will let your bet stand if your player retires hurt and others will void it. You might get lucky in this situation, equally you might lose both sides of the bet.
4- are you betting on the same fixtures? Can be easily done if you are betting on matches with 2 legs or if teams play in the league and in the cup around the same time.
That's it really, I generally wouldn't take an arb with a greater than 5% return because it becomes risky above that (there are exceptions but I'm not sharing my secrets!).
