Similar to a stock exchange.
Individuals trade positions while the operator merely facillitates the trade, much like a broker, for a set fee.
For example if I want to win $100 on Seattle+3.5 at -110 and you want to bet $100 on Chicago-3.5 at +110 then our bets can be matched and we have made a trade on the exchange market.
In practice a market maker is required to provide liquidity and an orderly market much the same as a stock exchange.
This is the essential concept...note how much it differs from a traditional book.