This is a very cool idea!

Obviously, it needs some work (especially judging from all the comments here). You already added an example to the homepage, but my feedback would be:

1. Add more examples of possible alerts. You can honestly just look at the Zapier homepage and copy how they show examples.

2. Include pricing information. I see the "create account" button, but I don't know how much you'll charge (If you don't charge, let me know if I need to add my API keys).

Still, it's a cool idea. It reminds me of existing tools that alert you to flight price changes, Amazon product price changes, etc... There's definitely something here.