Hi! I'm most interested in getting water. What are you looking for?
Hello, could you tell me why water is your most interest?
If something goes wrong on the camping trip and I get stranded, it seems by far the most useful thing to have around.
I understand. I have a need for water too along with firewood. 
Okay me too. Is there any reason you would need more food?
I don't need more food, I have a specific diet I must follow so I bought plenty of food. I would like 2 water and 2 firewood. What do you think about that?
That doesn't seem like a great deal to me because I'm left with only 1 water and 1 firewood. What if you get 2 water, 1 firewood, and 1 food; I get 1 water, 2 firewood, 2 food?
Ok, I can see how it does not seem like a great deal. Your counteroffer works for me 🙂
Okay! I will put in the offer and then you can accept. Everybody happy? 🙂
Yes, everyone is happy 🙂