Double-check inventory when checkout started

Problem/Motivation

On one site I'm involved in they regularly have people add items to their cart and leave them there for several days. It would be beneficial if the system could double check inventory when checkout starts, to make sure someone else hasn't snagged the item.

Proposed resolution

Double-check item inventory before the checkout process begins.

Remaining tasks

Work out the best solution to this problem.

User interface changes

When loading the checkout process, if an item has ran out of stock the visitor will be made aware of the change and checkout will be blocked.

API changes

TBD

Data model changes

TBD