This assumes that there is a chance to create a matched book. This is usually possible for something "liquid" like a big league sporting event, but not always f

This assumes that there is a chance to create a matched book. This is usually possible for something "liquid" like a big league sporting event, but not always for poorly-followed sports or weird prop bets on low-interest games. Even for liquid games, the book is not always perfectly matched and they may end up with some actual exposure to the outcome of the event. In any case, in this scenario its just one guy looking to make a bet. There is definitely SOME line, possibly million-to-one or higher, that a smart book (or any rational actor) should be willing to take the bet.