Which is the best way to evaluate the market demand / supply relation when it is NOT a stable market ?

Past data interpretation can help a bit if available and if same pattern remains till now.

 Statistical and quantitative models can also help to predict market demand and supply. The major challenge in every method is to find out the factor responsible for changing demand / supply. If such factor has been successfully been discovered than even in unstable market you can get more appropriate demand/supply forecast. 

Evaluating the elements that making demand/supply fluctuate, analysing seasonality and the cycle of demand/supply for previous period, anticipating customer behavior and reaction to that instabiliy. 

Have a look on the sales figuers for the last3 years (Monthly sales) it may help you