Lines move fast at Matchbook, it's really only useful if you're predicting line moves rather than reacting to them.
In this case (if you saw my earlier post before the edit, which I guess you did from the reply), it wasn't a true exchange but a "bet brokerage" firm, who get on at various (unknown to me) places up to the total volume, in exchange for a comission. Another idea from the people who brought you betting exchanges (Us Brits ;-))