Coding an app similar to NowInStock

Hey everyone,

I’m trying to build something that’s similar to the website Now In Stock.
It’s a service that updates you on the status of products you want to purchase online (specifically across Amazon, Walmart and some other sites). It looks like a useful tool, and want to challenge myself but not sure where to start with something like this. If you look at their FAQ, the founder said that they’re an average programmer (or at least were when first designing this site). So it can’t be too hard to build for someone with 2-3 years of experience right?
Any way, would appreciate any ideas. My initial instinct would be to use something like Selenium…