4Game

I recently did some calculations, since I got curious about the nitty gritty of how traders work. The first thing I did was confirm how each trader's buying rates worked.

Every item has a set ingame value that can be viewed by requesting that item in a barter trade. I also confirmed that each trader has a set rate that they will pay, as a fraction of that value. Here is an example showing Therapist's rate. While traders will only buy items from certain categories, the rate that they pay does not change for items from different categories. So far, this is community consensus.

The next thing I did, of course, was determine each trader's going rate.

^{Note that your prices WILL vary based on your Charisma – but as far as I know, they do NOT vary based on trader influence or level, so these comparisons should work relative to one another. Feel free to correct me if this is not the case.}

So far, so good. I repeated this process for Peacekeeper, multiplying his rates by 116 (his going rate for rubles per dollar), and got a multiplier of 0.585 (approximately). The reason why this is approximate is because Peacekeeper will round to the nearest dollar, meaning that his rates will vary somewhat for small sales, and approach 0.585 the more stuff you are selling.

So what does this mean? Well, Peacekeeper will only ever short change you by a dollar at most (more likely 50 cents depending on how it's rounded). Mathematically, this will never outweigh the difference between his and Mechanic's rates once you pass around ~4000 rubles in sale price – so not much at all. If you ever buy dollars, and you are selling to Mechanic, you are losing money compared to just selling to Peacekeeper.

TL;DR: Peacekeeper's rates are better than Mechanic's and he buys all the same items (more, in fact). Unless you literally will never use the dollars, you should sell to him instead, but be careful not to sell in small increments since he may round down to the nearest dollar.

Also, I made a little graphic. It's nothing special since I mostly made it for my own purposes, but I figured I'd include it in case anyone else would find it useful.