The answer to your questions is Price2Spy - a state of the art tool used by many online retailers worldwide (Price2Spy is currently been used to monitor more than 1000 Web stores worldwide).
Once you tell it which competitor products it should monitor, Price2Spy will keep checking that Website and alert you of any price / availability change it notices. More than that, Price2Spy offers you a number of comprehensive reports enabling you to analyze your competitor's pricing policy in any given moment in time.
Price2Spy offers 30 day free trial for all newly registered users.
Due to sensitive nature of Price2Spy services, we are unable to disclose any clients / references.