Tit for tat
Very rare3.5%ObjectiveOptional
Sidorovich's worst nightmare: buy and sell goods for the exact same amount of coupons in one deal.
This is quite easy; trading with vendors works in the same way as many games. Simply select some items to buy, and some items to sell, such that they amount to the same value and no coupons change hands when you confirm the deal. This can be done with one piece of ammo and one piece of equipment, for example.