Both Fairlay (click for link) and BetBTC (click for link) are sportsbooks and also betting exchanges, though I've personally never used either of them and they don't seem to have a lot of volume from what I can see right now. I don't currently see any active scam accusations against either site so they should be safe to use (BetBTC's thread dates back to Nov 2014 and Fairlay's to July 2014), though please feel free to correct me if I am wrong.