A plural form of a noun usually implies a set having more than one member of the same type.  For example, a reference to "dogs" is understood to refer to more than one dog.  No one understands a reference to "dogs" to mean a dog, a cat and a mouse.  That is not necessarily the case, however, under the California Corporations Code.

Section 29005 of the Corporations Code defines "commodities" to mean "anything movable that is bought or sold".  Section 29504 assigns a much broader definition to the singular term "commodity":

“Commodity” means, except as otherwise specified by the commissioner by rule or order, any agricultural, grain, or livestock product or byproduct, any metal or mineral (including a precious metal set forth in Section 29515), any gem or gemstone (whether characterized as precious, semiprecious, or otherwise), any fuel (whether liquid, gaseous, or otherwise), any foreign currency, and all other goods, articles, products, or items of any kind.  However, the term “commodity” shall not include (a) a numismatic coin whose fair market value is at least 15 percent higher than the value of the metal it contains, or (b) any work of art offered or sold by art dealers, at public auction, or through a private sale by the owner of the work of art.

Putting these two definitions together, it is possible for a multiple items to be "commodities" even though a single item is not a "commodity".  For example, a numismatic coin of the requisite value would not be a "commodity" even more than one such coin would meet the definition of "commodities".   The explanation for these seemingly inconsistent definitions is that they are found in two different laws.  "Commodities" is defined in California's Bucket Shop Law while "commodity" is defined in the California Commodity Law of 1990.