Pardon me if I'm not thinking about this in the right way, but how would you hold the store owner accountable for refreshing or updating their catalogue on the app if they don't already use an inventory management system for purchasing and re-stocking? Isn't that a huge shift in behavior?
What normally happens is that availability is a boolean yes/no, and gets set to no when the first person asks for something that's run out. The merchant then sets it back to yes after stock comes in. That's a lot less work than integration ERP systems, and achieves the same result with the collateral damage to a few customers' experiences.