It is called The Law of Supply and Demand. I'm sure there are zounds of articles or videos on it.
In the model, the price of a unit will change as you adjust either supply or demand. If there is lots of demand and little supply, the price is high. If there is lots of supply, the demand cannot force the price higher. Assuming the seller wants to sell, they have to compete with other sellers. The more units for sell, the less negotiating power they have because someone more desperate can sell cheaper.