This is pretty cool. I did something similar with adwords scripts a few years ago. I was doing facebook advertising a few years ago and the ads featured products that would occasionally sell out, but there wasn't a way to monitor them so if something sold out, the ad would go to an empty product page. I rigged up a system that would take a list of URLs, look for "sold out" on the content of the page and then send me a text if it found a sold out product. I used Adwords scripts because it was able to run jobs hourly and all I really knew at the time was JS. It was a fun little project.