I like this concept (in fact, it's one I've been playing around with in my head for a while) but there are 2 major blocking-points you'll have with user-adoption:

1) People won't want to give their amazon credentials out to what may simply be a fly-by-night startup. Until you get some semblance of reputability, you'll have a hard time with UA.

2) Amazon could very easily write in a feature like "buy when < N dollars" and completely invalidate your startup.

Anyone have any ideas as to how to combat these issues? Would the general HN populace be interested in a service like this for say, either 10$ a month or X% of what you saved as a result of using the service?