What are the advantages of using demand systems estimation? How is this different from individual demand estimation for product?
One advantage of considering demand systems is that more data is available for identifying the degree of income effects and substition between commodities. A second advantage is the possibility to impose and test microeconomic restrictions (which are between equations). An introduction to the estimation of demand systems with an historical perspective is provided by:
Berndt, E. R., 1991, The Practice of Econometrics: classic and contemporary, Addison Wesley.
More recent treatments are available in most econometric textbooks.