How can I notify store owners when stock levels are getting low? For a Craft Commerce site, the store owners wish to be notified automatically when a stock is starting to get low. Can this be achieved with a plugin, and if so, how?
Just like Craft itself, Craft Commerce has lots of events you can have a plugin respond to. In this case, you'd want to write a plugin that waits for commerce_orders.onOrderComplete (per documentation) that evaluates the stock levels and probably uses Craft's Email Service to fire off notifications to the store owners.

Here's some quick pseudocode:

&lt;?php

namespace Craft;

class StockNotificationPlugin extends BasePlugin
{
    ...

    function init()
    {
        craft()-&gt;on('commerce_orders.onOrderComplete', function($event) 
        {
            // loop through the items in the order ($order-&gt;lineItems), 
            // checking the remaining stock ($item-&gt;purchasable-&gt;stock)
            // against your desired threshold, firing off notifications
            // if applicable
        }
    }

    ...
}