They are products that are sold by a fair price because they are basic and generally necessary, especially household supplies. Basically all sort of groceries and shops buy and sell the same product for the same price.
Simple answer is Supply and Demand balance. If supply is more price goes down , demand is more price goes up.
for any commodity movement get these number PIS=/=ExC , P=production, I=Import S=Stock...
It's actually the other way around: the supply of a commodity influences its price, in that the more of the commodity you have, supposedly the lower the price to get people to buy more of it.