@ 9:10 am 1/21/2018 i have encountered the trader bug again where you cannot buy ammo and weapons or sell, this is caused by in the menu perk selection where the perk you are using is not highlighted but is active and you can see the selection bar on another perk but not selected. This is preventing the current Perk to be invalid, to by pass this issues enter the perk selection and reselect it this will be valid once you exit and will be able to buy ammo, weapons and sell again.