To have demand

what must you have

The desire and the ability to pay for the item

Desire, willingness, and ability to buy a good

Explanation:

Many things change the demand for a given product or a service. The change on the underlying factors will determine the quantity that is needed and on what prices they will be buying at. Therefore this will cause a shifting in the demand of the given product.

The willingness of a person to purchase brings the desire depending on the tastes and preferences. When somebody does not want something, they will not buy. Therefore the ability to purchase shows that income is crucial.