Commodity is not a share market thing at all - but the share markets offer commodity contracts to buy or sell commodities. A commodity is just something that may be traded (bought sold). Gold, Wheat, Orange, Pork Bellies, Diamonds, Energy are all commodities. Merchandise is items (commodities) specifically for sale. Goods are moveable things, like 'household goods' or things someone has undertaken to supply or deliver (moveable) - pretty much the same as Merchandise.

Ok commodity is a stock term meaning a share of gold, soybeans, corn etc.. In slang someone or something could be a hot commodity meaning they are rare or valuable Goods and merchandise are the same thing except goods are plural. Merchandise single or plural. Goods not used much in regular speech unless the term goods and services which usually means the price includes the product and other services. Also "the goods" is old movie slang" for something illegal. Another use for goods is the slang word goodies( which means usually snack foods, sweets,chips) anyone says you want some goodies usually means snacks.