The answer that should have been given is that central banks ( Cartel, or what ever ) will never sell all their gold. They are not that stupid.
What they will and do sell is what they deem to be a reasonable risk to maintain control for as long as they can.
A major part of that scheme is the diversion of investors from the physical into the 'Paper' gold.
But sooner or later, and it will probably be much sooner, buyers realize the game and demand the turn over of the real thing in volume. That is when the jig will be up and it will arrive like an avalanche. Not unlike the failure Of Lehman Brothers triggered a change in the financial fiascoe.