Players will be able to trade both with other players and NPCs. The dynamics of economic system includes variable factors that are out of the player’s control. Floating exchange rates, shortage of goods, even trader’s health - all of this is controlled by the game AI.
NPC-traders will work in different currencies. You can exchange one currency to another by trader, but with a small commission.
There will be a number of factors determining the standing with traders - their affiliation and sympathies, your general reputation in Tarkov, how often you trade with them, services you perform for them, and many more. However, once gained, standing won't deteriorate over time only. I.e. if the course of you actions did not influence your standing, it will stay the same no matter how long you don't visit.