You will probably want to analyze the stock data to give an estimate of how long until it is depleted, as well as traffic analysis to calculate if there is time to drive to the store before the stock runs out.
Example: You have a PHP script that does all the 'heavy lifting' and analysis of the 'quantity' of products...
Then you can return whatever you like back to the Arduino/request.. to parse and display/show/indicate how you like the returned data..
What part(s) are you stuck on?
1.) Do you have an 'end point' this ESP module is supposed to connect to?
2.) Do have the web scraping/comparing script already written/tested?
3.) Have you written any sort of protocol to parse the incoming data from this web request?