Restock monitor

I need you to develop some software for me. I would like this software to be developed for Windows. I need a program that i would sell (i would need to be ableto generate activation keys for users) the program will basically monitor websites for change (similiar to the google application distill) basically users need to be able to input links and the program will monitor the link and once the program notices a change in the link the user will be notified by text (ex: a link for a pair of sold out sneakers and once the sneakers restock a text message will be sent to the user allerting them).