How might the law of supply and demand affect an individual seller? How might it affect a buyer?

The law of supply and demand is basically this: The value of something is commensurate with the demand of that something with the availability of that something.

Take widgets. You have widgetA and widgetB.

Not too many people want widgetA; there's not a high demand for widgetA. Also, there are a lot of widgetA's available for purchase.

Now, on the other hand, lots of people want a widgetB. Also, the number of widgetBs produced do not keep up with the demand for widgetB.

Considering the top two hypotheticals: Joe sells widgets. He stocks both widgetAs and widgetBs. Because his supply of widgetAs exceeds the demand (number of customers who want to buy them) he will be forced to sell them for less than he could sell widgetBs because his supply of widgetBs is less than the demand for them.

The higher the demand, the lower the supply, the higher the cost.

The lower the demand, the higher the supply, the lower the cost.