Two bodies rapidly cooling, another one slowly bleeding out. A burned out car, a wrecked 8x8 combat vehicle, and a limb from an Eisenhower power suit.This is why the first cardinal rule of making a trade is "don't try to shoot your way to a better price". Of course, the second rule is - "presume the other guy will attempt to do so".
Stopping to replenish supplies is one of the major sources of actual role-playing in an External Style Game. Most characters might expect to spend long periods without seeing another party or settlement
Negotiation often begins well before the topic of price even comes up. Convincing the others that it is worth their while to stop and see if the party has something worthwhile is the first challenge. After that is choosing a place for the meeting.On one hand - choosing defensible terrain can give the advantage if a firefight should break out, but also decreases trust, making it more likely one will. An open area is a sign of more trust, but can be dangerous. Meeting in a settlement with moderators and weapons checked at the entrance is a good middle ground - provided one is anywhere nearby.
Once a time and place is settled on, there is always the matter of price. The black market doesn't feature menu boards after-all.
This is also a good time to pass along rumors about what is going on in the zone. A place once known for high reanimate activity might become oddly quite for a while, or Citizens might start another offensive. City states and settlemants make periodic statements about how they're doing and offers for what they need - opening up a chance for some payment or at least direction other than wandering.
Nor should the GM pass up the chance to add some other fun scenarios. Perhaps a player turns out to actually know or is related to one of the people in the other group. Or a reanimate ambush forces both sides to work together, resulting in a fire forged friendship that will ease further deals.