Google’s Matt Cutts answered in a video what webmasters and site owners should do about their out of stock products on their e-commerce sites.
Matt Cutts basically said it depends on the size of the e-commerce site. He broke it down into three sizes: small sites with tens of pages, medium sites with thousands of pages and massive sites with hundreds of thousands of pages or more.
Small E-Commerce Sites
Small sites that sell items, such as handmade furniture, that showcase a product that is out of stock should likely link to related products. This way the customer can see that this owner can make or design something as displayed but at the same time, show other products that are currently available in stock that the customer can purchase today.
Of course, it may make sense to add a manufacturing time next to the items that are out of stock.
Medium E-Commerce Sites
The normal, medium sized, e-commerce site, that sells thousands of products, and where some of those products are out of stock. In that case, the site owner should 404 – page not found – the products that are out of stock.
That is unless you know the date that the products will come back in inventory. If you know when the products will come back in inventory, inform the customer on the site and let them choose if they want to order it for later delivery or not.
Otherwise, 404 the page because it can be frustrated for a customer to land on a product page that they cannot buy.
Large E-Commerce Sites
For really large e-commerce sites, with hundreds of thousands of pages, such as Craigslist, you should set the date the page will expire using the meta tag, unavailable_after tag. This way, when the product is added, you can immediately set when that product page will expire based on an auction date or a go-stale date.
This information is treated as a removal request: it will take about a day after the removal date passes for the page to disappear from the search results. Google currently only supports unavailable_after for Google web search results.
Here is the video: