He was Trading for the gas and bullets. I think he was offering Milk, Water would have made more Sense imo.
Joes back story From the Mad Max Wiki, source is the comics IIRC
Immortan Joe - formerly known as Colonel Joe Moore - was a veteran of The Oil Wars and a hero of The Water Wars. He formed a biker gang terrorizing the people after the fall. His gang included henchmen dubbed Major Kalashnikov (later known as The Bullet Farmer) and a strategist: Deepdog. With each battle the gang grew stronger by slaughtering the leaders of other gangs and taking the women. With that, his idea of bringing back society willingly took a dark turn. Eventually he and his gang entered the wasteland, where he would learn about a massive aquifer plant later known as The Citadel.
Out in the wasteland, Colonel Joe Moore spotted a group of rogue MFP officers (Roop Sarse & Charlie) killing a group of people. He sent out a war party that found a survivor (later known as The People Eater) with knowledge of the location of a massive aquifer built into a series of natural stone pillars in the wasteland, sitting on a lake of fresh water. After learning about the location of the aquifer, Colonel Joe Moore tried to bargain with its inhabitants, wanting to trade the captured women for water. His offer was turned down and his gang was attacked. He decided to look for water elsewhere.
Colonel Joe Moore sent scouts to look for water, but what they found was a derelict oil refinery and an abandoned lead mine. He also sent out disposable members of his gang to climb and inspect The Great Rock, but all of them were shot down
The harshness of that world blurred the edges of reality. People worshiped Joe like a god. Meanwhile Joe rebuilt the aquifer into The Citadel and scoured the land for resources. He reclaimed the Oil Refinery up North and used it to produce guzzoline for his armada. Dubbed The Gas Town it was run by the captured fat man who tipped Immortan about the location of the aquifer and he became known as The People Eater, reckoner human calculator.
Immortan Joe refurbished the abandoned lead mine and turned it into The Bullet Farm under the rule of his former gang mate, Major Kalashikov now known as The Bullet Farmer.
The legend of Immortan Joe captured the hearts of many and they traveled to The Citadel to seek for better life. They were known as The Wretched or The Triumvirate, standing at the lowest in the hierarchy, occasionally allowed to drink water from Immortan Joe's supplies. Only the healthy ones were taken by Immortan to become either Milk Mothers, Treadmill Rats, War Boys or Breeders. None of them knew what fate awaited them. The young boys were forged into fanatical believers of Immortan Joe's cult, becoming mechanics, worshipers of the V8 and Immortan Joe's greatness. However, Immortan Joe was fearful of his future seeing his sons unworthy of becoming his successors. He created a 'breeding program' in order to produce a healthy successor for his empire. He sought the healthiest and purest women and called them "The Five Wives". The Wives escaped his tyranny with help of Imperator Furiosa.