If you are running a Pay Per Click campaign and a particular ad leads to a product that is out of stock, you are throwing money down the drain. The potential customer has clicked on the ad, arrived on your site only to find it is out of stock, and will then, more than likely, leave. Not only has this cost you the price of the click, but potentially it has cost you brand reputation in the mind of that user.
Whilst for some companies it may not be a simple or cheap matter to tie your inventory directly into your PPC campaign management, and automate the process when a product goes out of stock, it is only a matter of moments to do it manually. Put on hold any ads which relate to out of stock products and make it a priority in your daily tasks.
The click may only cost you a small amount but these can add up over time, and are so easily avoidable. And also, if a product is out of stock, make sure you offer your site visitor alternative products, or a chance to be informed as soon as the product comes back in stock. Harvesting email addresses like this will give you the chance to pick up potential customers in future even if the item they wanted at that moment is not available.