Alright, new discovery today.
You can lower a shopkeeper personality if you want a slight discount.
A lowered personality results in lower Barter after all.
So if you want the
best prices, you should consume everything that buffs your Personality & Barter while giving the Trader everything that lowers Personality & Barter (Onion, Alcohol etc)
But that's not enough, you'll notice the best traders have the best Barter skill, therefore the
worst prices for you.
Compare the gun store owner inside the Commerce compound with the market folks just outside in Krasno. There's just no contest. The only problem is the market people have less cash to pay you with.
How to solve this? Well, the first option is to find a locker in Krasno and stuff your unwanted loot inside, wait a while - these stores replenish cash over time. Gradually liquidate the goods.
The second option is more random, carry a few valuable loot while traveling in hopes of running into trader caravans. They usually pay more for your goods and sell their stuff for less.
Or do both at the same time.