I got idea: Caravan Masters in Caravan Encounters could sell resources like junk, ores or parts. Each city should have caravan with different items, for example:
Redding: Iron Ore, Minerals, small chance for HQ Ores
Boneyard: Junk
San Fran: Electronic Parts
and so on. Amount of items should not be too much, so players won't kill whole caravan just for few iron ores. Apllying this will encourage interaction with caravans other than "kill them all, loot the guns". I'm waiting for Your opinions.