Some of this must be down to the methods used to display the quantity available. Probably three ways this figure fluctuates?If everything is 'selling out' on the night, and then it's available again the very next day, it means people are cancelling their orders before the midnight 'cut off'.
If people are cancelling their orders, then it must mean only one thing - the prices are higher than people expected to pay.
Take note Gemporia!
1. Phone: If you telephone Gemporia and select the option to indicate that you're calling about the item on screen, I assume that this deducts 1 from that total as a reservation (or "allocation" I've heard used by presenters).
2. Online: No idea if adding to a basket online does the same?
3. Cancellation: abandoned 'phone calls, baskets not checked out, payment collections denied
Either way, you're right. A "total sellout" one minute, with a hefty quantity available again a short time after or the next time the item is auctioned indicates a lot are in the third category.