Mountain View, California–Chief Webspam engineer of Google, Matt Cutts, answers a question in a Webmaster Help video about what to do with webpages which feature products that are no longer available.
Cutts states inactive versus active product availability does matter, but there are ways to deal with such a situation.
Cutts says in the video below that product availability in terms of time also matters as well as the balance between products which remain available for purchase on an eCommerce site and those that expire. The size of the selling site plays a large factor in dealing with inactive or unavailable products.
Dealing with Non-Available Product Pages
Cutts suggests that small eCommerce sites redirect visitors to other pages, such as like or similar products, or products which might interest potential buyers. However, this strategy changes for mid-sized eCommerce sites with hundreds or thousands of pages, in which case the engineer states it’s better to use a 404, because “you don’t want to be known as the product site that whenever you visit it, you can’t buy [the product] anymore, because users get just as angry as getting an Out of Stock message…” going on to elaborate that Google recognizes “unavailable after” meta tag.
The unavailable after meta tag is one which tells search engines to stop indexing a page after a specific date. Cutts states that using this is more preferential than other methods of dealing with non-available product pages on an eCommerce site. However, if a product is only temporarily unavailable, Cutts recommends leaving the page up and letting visitors know the product is currently out of stock. The use of the unavailable meta tag should not invite a duplicate meta penalty.