Did not find this my self, found it on youtube.

This is a duping glitch, BE shouldn't but might ban for it so be careful and don't do it so you have insane amounts of money.

1.get a backpack, any is fine
2.go to general store
3.buy binoculars
4.put the binoculars in your CLOTHES not backpack
5.buy NVG's
6.put NVG's in backpack
7.Sell binoculars
8.each time you sell binoculars the nvgs will double
9.drop binocs when ready to sell nvgs
10.profit