See our Product webmaster docs for full explanation of the Offer type, and how to encode Product data in RDFa, microformats or microdata. This page exists to contain a machine-readable microdata description of the Offer type itself (at the URI

An item with the item type represents a offer. The following are the type’s defined property names:

Property Description
price The price of the product. A floating point number. In addition to including the price in the content of the tag itself, you can also use the optional content attribute to specify the price to Google in machine-readable format (in this case, you must use only a decimal point as a separator).
currency The currency used to describe the product price, in three-letter ISO format.
priceValidUntil The date (in ISO date format) after which the price will no longer be available.
seller The seller of the product. Can contain a Person or Organization.
condition Any text may be specified. If the condition attribute is used, the value of the content attribute must be one of the following recognized values: new, used or refurbished.
availability Any text may be specified, but the value of the content attribute must be one of the following recognized values: out_of_stock, in_stock, instore_only, or preorder.
offerURL A URL to the product web page (that includes the Offer). (Don’t use offerURL for markup that appears on the product page itself.)
identifier The product identifier. Google recommends including brand and at least one identifier for each product. Recognized types include: asin, isbn, mpn, upc, or sku.
itemOffered The item being sold. Typically, this includes a nested Product, but it can also contain other item types or free text.