first dude: "hey i have way too many potatoes, here is some".
second dude: "i have too much milk, here is some."
First dude: I have too many potatoes, but sure would love some milk. Want to trade?
Second dude: Sure, I have way too much milk, and I'd love some potatoes. What do you say to 5 potatoes per gallon?
First dude: It's a deal, and any time you want more, just come on by.
Of course, this is inefficient, and there's no guarantee that the guy who has milk wants potatoes. Maybe the guy who wants potatoes has corn, and the guy who has milk wants corn, but not potatoes. That's why we invented money. To facilitate trade.
simpler solution: put it one big pile, take what you need.