I gather that the measurement of GDP is with anything that is a final good or service. But this does not include second hand good or intermediate goods which are good that are used to make or build other products. I believe that the informal market transactions may be a limitation as well because the GDP underestimates the actual amount of output produced in the economy, but I am still trying to understand what the informal market transaction are. I understand that the GDP is consumption+investments+government purchases+Net exports, and anything that does not fall under those catogories would be a limitaion?