What is one atthe most important factors todetermine how muchof a product people buy or...

Theprice levelis one of the most critical factors influencing how much of a product people buy or sell in a marketplace. According to the laws of supply and demand, changes in the price of a product directly affect consumer behavior, where higher prices typically reduce demand, and lower prices increase it.

References:

Volume 1, Chapter 4:Overview of Economics, section on "The Market" discusses supply, demand, and how price levels determine market activity​.