Betfair, TradeSports
If you already have several outs, sure, investigate what the exchanges offer. But I advise you to not depend upon Betfair and Tradesports too much. For some of the more popular wagers, like futures on major sporting events, check them for better odds. For most day to day American sports wagers, you might not find anything there.
One thing I read is that much of the activity at the exchanges (right now) is involved with Soccer and sports more popular in Europe.