I am curious how you will deal with this problem: lets say I am at the store using InStock, and then someone takes the last package of chicken. So, I report there is no chicken. Another user approaches and also reports there is no chicken, so there must be no chicken! After we both leave, the stock boy comes and replenishes the chicken.
What will motivate reports?
Probably nothing really. It will work a while for frequently out of stock items when the app has a small number of users. This replaces text messaging to let others know what's in stock.
But with many users, notifying other people will just make the store run out of stock sooner. So people will revert to texting their friends to let them know when something's in stock.
I'm curious what would be motivating here, too. Of all times to dilly dally in a grocery store, now's not the time. I want to get in and get out to reduce exposure time--not spend time updating an app with what I see on a shelf.