Hello, how are you doing today?
Very good. How about you?
I'm doing well. I'm looking forward to going on my camping trip but will need some additional items to make sure the trip is safe, healthy, and enjoyable. 
Same here. 
I have diabetes, so it is very important to me that I get extra packages of water and food. When I'm at home, I always need to pack extra drinks and snacks for myself in case I go into shock and need to take medication. The trip could be very dangerous for me if I don't have access to these additional supplies. 
Actually mine own purposeful need is firewood and water. Being on a first time adventure, I lack these items as I was inadequately prepared.
Okay, so firewood and water are your two most pressing needs?
Yes.
Okay. I'm willing to give in on the firewood, but I really can't give in on the water or the food. As a diabetic I have to put my health first. 
I can give out the food in exchange for the firewood
Okay. I need all of the water, if you're getting the firewood. 
I can't give out all the water. I need to be hydrated as well. 
You already have all the basics, though, so you won't lack sufficient water to be hydrated. I have a serious medical condition that requires that I have access to additional water. 
The maximum bundle I can give is all packages of food + 1 package of water
Sorry, that won't work for me. I've already said that I have a medical condition that requires additional water. You do not need that much water just to be hydrated -- you already have basic water. I can give you all of the firewood, but I need all of the water. 
In that case I exchange all the water only for all your firewood
I can give you all the firewood - but as a diabetic, I still need at least one extra package of food for my blood sugar levels. So, as my final offer I can give you all of the firewood, and two extra packages of food. Do you agree to these terms?
No, I give out only one extra package of food
I think you misunderstood what I said. I said that *you* would receive all of the firewood, and two extra packages of food. 
ok
Ok, great. It was nice negotiating with you.
same here