When do you say a product is a commodity?

Answer 1

A product sold only on price, because the different sellers of it offer equal and interchangeable versions of it, is a commodity.

Let's say you, I and the supervisor of this forum all grow dent corn (the kind you make cornmeal from) for sale. There is not much, if any, difference between any two farmers' dent corn if it's grown in the same area, so I really can't go to market saying my corn's better than the supervisor's. We are therefore selling a commodity item.

OTOH, if we convert the corn into a product like tortillas, we no longer have a commodity item because there can be significant differences between two brands of tortillas.