A common question large sites or e-commerce sites have to ask themselves is what do I do about product pages that are either temporarily out of stock or forever out of stock. It is a question we asked a couple times. In fact, Google's John Mueller gave his two cents in 2008.
Well, now, Google's Matt Cutts created a short video answering what you should do in three different situations.
As I summarized at Search Engine Land, Matt Cutts said it depends on the size of the site.
- Really small sites can show related products on a product page that is out of stock
- A medium site needs to 404 the page that is out of stock or if it is temporarily out of stock, communicate when it comes back
- Huge sites should use a meta unavailable_after tag to automate this process when you know the product will go out of stock, such as craigslist.
That is the short set of answers and the big question SEOs are asking, why not 301 a dead product to a related product? Well, clearly Google is telling you not to do that. John actually explained why in 2008, in short, it can be confusing when someone is looking for product A and is being shown product B.