1. When a customer adds an item to their cart I want to reduce the stock availability immediately for any other customers who may visit that item
(whilst the first customer is still in the store and hasn't checked out yet).

2. I want to be able to make the stock that has been allocated to a cart that has been abandoned available again to other customers.

Let me explain.

If I go into a supermarket and I take the last tin of beans off a shelf and put it into my cart then other customers see an empty shelf however the store inventory will still show one tin because I haven't yet paid for my beans.
The tin is unavailable but still in stock.
When I pay for the beans then it will be removed from stock.
If I abandon my cart then an employee will put the tin of beans back on the shelf and it will become available again to other customers but the inventory will show no change.

Any ideas on how to make an item temporarily unavailable whilst in someones cart and how to determine an abandoned cart would be greatly appreciated.