In the design of the registry, we need to choose what periodicity people receive their charge  to allocate towards utilities.  This gets really interesting, and somewhat technical, so let's start with an example.

Let's set this amount of time be every 10 minutes.  Every ten minutes, everyone gets 10 minutes worth of charge to decide what they want to allocate it towards. 

No one is going to login to the registry every 10 minutes and decide what to do, so they will configure their account to automatically trade with their preferred null utility if not allocated before the 10 minutes is up.

So now it comes a questions of - when do I exchange my time, and with whom?

No one is going to want to figure this out, but the Registry could easily do this.

There are many different algorithms that can be choosen to optimize this exchange of unneeded charge for money.  Irrespective of the algorithm, all of them are deciding the same thing: "Who gets to trade first, and with whom".  To maximize the exchange, everyone will post at the same time until the supply is maximized.  Next, all exchanges have to occur, in some order, each transaction reducing the supply for the next.  The first pair that trades their unused time first is going to have an advantage over all others as the supply withers down to only 2 individuals left, the last (if any) is left with no one to exchange with.