Since it is by definition a low (sales) volume process, you'd only need to keep one or two of each thing in stock at the Etsy fulfillment center. Items in stores run on "tight logistics" would just have their items constantly going in and out of stock as they alternate between 0–2 copies in stock at the warehouse.

Of course, if things were constantly going out of stock, then you'd want to create a system to allow people to put down a hold on their card to reserve a copy of something, waiting in a virtual line to be the first to get it when it does come back in stock. (Which would actually work much better than Etsy does today, as it would give makers the ability to know how many of each thing to target making as a batch to attempt to satisfy upcoming demand — without money already captured forcing them to work to a potentially-impossible deadline.)