Bob, Sue, Jane and Karl have 24 hours worth of charge to allocate towards utilities for the day, and have a balance of 12 hours in exchange currency to spend towards utilities.  They all have nothing worth doing for the day, so decide to exchange on their null utility.

Bob allocates his 24 charge towards the null utility and posts it for sale on the registry. Sue, Jane and Karl do the same.

Bob is first, and buys Sue's null utility for T / S = 24 / 4 = 6 dollars.  Sues 24 charge is converted to 24 dollars.  So now, Bob has 12 - 6 = 6 dollars and Sue has 36.

Next is Sue.  She buys Bob's null utility (perhaps because they are friends) for T / S = 24 / 3 = 8 dollars.  Bobs 24 charge now becomes 24 dollars.  So now Bob has 30 dollars and Sue has 28.

Next is Jane.  She buys Karl's null utility for T / S = 24 /2 = 12 doĺlars.  Karl get 24 dollars from his charge.  So now, Jane has 0 dollars and Karl has 36. 

Finally, Karl.  He buys Jane's null utility for T / S = 24 / 1 = 24 dollars.  Jane gets 24 dollars.  So now, Jane has 24 dollars and Karl has what he started with, 12.

As you can see, becaues Karl was last, he wasn't able to get any benefit of this exchange, so the order is important.  However, this is just the simplest method of exchange, there are others.