Didn't Avalon also state that Batch #2 was going to be 500 units? I took "build to order" to mean they will not order the parts till they have been paid..not to mean that they will build as many as everyone wants.
I think it means that they will build what the customer orders, in the quantity that the customer requires, and not a single unit more. The 500 (600?) cap is because of capacity in the delivery time window. If they only get orders for 100 units, they will only make 100 units. They don't keep inventory of finished product, only parts. Think of it as the difference between a shipyard and a toy company - the shipyard doesn't have a stock of finished ships to sell you unless one of their customers had a bankruptcy. By only accepting bitcoins, they're not exposed to that risk.
I'm sure one person could order 1000 units if (s)he wanted, but only the first 500 would be delivered in the time window.
Edit: Actually, the idea of reducing inventory and storage costs has trickled down even into furniture sales: When you buy an expensive custom sofa, it hasn't been built yet, or is already on the truck from Poland.