Many different ways ill list the ways of looting Easy, Medium,Hard and extreme
Easy: not the most efficient way is to loot militarized zones including NWAF,NEAF,Balota and any other place now this is not the best method but if your a solo player looking to make money you can easy walk off with 20-30k+ worth of gear in a car.
Medium: This is looting radiation zone, there are not many people that go to this area you are required to have a gas mask from the black market. with a helicopter you will average 30-40k+
Medium: Doing missions(Not checkpoints) if you're the first