Good idea but the algorithm of “in stock” is not precise. Depending on how the system is designed, the value returned can vary making it hit and miss. As an example, imagine you searching an item using the filter, and the system finds one in a local store. At that precise moment, someone walks into that store and buys that item. You click “reserve” but the system may now say no stock. There are dozens of other scenarios many dependent on the technology used.
to minimise such errors, most systems will only check upon you pressing buy or some times putting it into the basket. Even then, you can have situations when click and collect can suddenly become next day delivery.