Product Advertising API
Developer Guide (API Version 2013-08-01)

Using the ItemSearch Availability Parameter

The Availability parameter enables you to search only for items that are available. The only valid value for Availability is "Available" as shown in the next example. Service=AWSECommerceService& AWSAccessKeyId=[Access Key ID]& AssociateTag=[Associate Tag]& Operation=ItemSearch& Condition=All& Availability=Available& SearchIndex=Apparel& Keywords=Shirt &Timestamp=[YYYY-MM-DDThh:mm:ssZ] &Signature=[Request Signature]

Items can become unavailable quickly. Even though this parameter filters out unavailable items, checking that an item has an OfferListingId is still recommended.