The economic factors affecting the price and availability of a commodity or product in a free market

'Market forces' is a fairly recently coined expression, which first saw the light of day in the economic rebuilding phase which took place after WWI. The earliest reference that I can find to it is in a piece in the US newspaper The Daily Northwestern, February 1928:

In these days of commercial agriculture the prices of farm products are the result of a combined action of a number of factors, such as supply, demand, market information, quality, seasonal production and industrial conditions. These factors are called market forces and their action determines the price at which farm products move.