From Wiki of the E-Business and Web Science Research Group

GoodRelations is a standardized vocabulary for product, price, and company data that can be embedded into existing static and dynamic Web pages.
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Modeling Inventory Levels / Available Quantities

In GoodRelations, that there are two different GoodRelations properties that refer to quantities of items:

• gr:amountOfThisGood and
• gr:hasInventoryLevel.

gr:amountOfThisGood is attached to gr:TypeAndQuantity nodes and indicates the quantity of the item included in the particular offer. In 80% of the cases, this should be 1.0 (i.e., "1.0"^^xsd.float), because the price etc. is meant for one item. A value other than 1.0 is appropriate when you sell a bundle of items or use units of measurement other than "item/piece", e.g. selling

• 100 tiles,
• 500 g of butter, or
• 1 camera plus 2 memory sticks.

gr:hasInventoryLevel, on the contrary, can be used to indicate inventory levels. Note that while gr:amountOfThisGood is an owl:DatatypeProperty, gr:hasInventoryLevel is an owl:ObjectProperty with a range of gr:QuantitativeValueFloat, because this allows for using arbitrary units and also specifying ranges.

It can only be attached to instances of gr:ProductOrServicesSomeInstancesPlaceholder.

So if you want to indicate inventory levels, use gr:hasInventoryLevel in combination with an instance of gr:QuantitativeValueFloat.

Example: Assume you have 80 pieces of a SCSI controller card on stock. Then, use the following pattern:

RDFa:

 
<div typeof="gr:ProductOrServicesSomeInstancesPlaceholder" about="#myProduct">
    Product Name:
    <span property="rdfs:label">Personal SCSI 16-bit SCSI Controller</span>
    <br/>Description:
    <span property="rdfs:comment">This low-cost, high-performance SCSI controller allows you to connect up to
    seven professional mass-storage devices to your computer.</span><br/>
    EAN/UPC code:
    <span property="gr:hasEAN_UCC-13" datatype="xsd:string">00010363780</span><br/>
    <span rel="rdfs:seeAlso media:image foaf:depiction">
         <img src="http://www.heppnetz.de/searchmonkey/pscsi.jpg" alt="Product Image"/>
    </span>
    <span rel="gr:hasInventoryLevel">
         <span typeof="gr:QuantitativeValueFoat">
              <span property="gr:hasValueFloat" datatype="xsd:float">80</span> items on stock.
               <span property="hasUnitOfMeasurement" datatype="xsd:string" content="C62"/>
         </span>
    </span>
</div>
 

N3:

foo:myProduct a gr:ProductOrServicesSomeInstancesPlaceholder;
  rdfs:label "Personal SCSI 16-bit SCSI Controller"@en ;
  gr:hasEAN_UCC-13 "00010363780"^^xsd:string ;
  gr:hasManufacturer <http://www.heppnetz.de/searchmonkey/product.html#heppcomputer> ;
  rdfs:comment "This low-cost, high-performance SCSI controller allows you to connect up to seven professional mass-storage devices to your computer."@en ;
  rdfs:seeAlso <http://www.heppnetz.de/searchmonkey/product.html>, <http://www.heppnetz.de/searchmonkey/pscsi.jpg> ;
  foaf:depiction <http://www.heppnetz.de/searchmonkey/pscsi.jpg> ;
  gr:hasInventoryLevel
      [ a gr:QuantitativeValueFoat;
        gr:hasValueFloat "80"^^xsd:float ;
        gr:hasUnitOfMeasurement "C62"^^xsd:string
      ] ;
.