"Stardock/WC pay nothing for the skins on this site. The tire dealer has to buy his tires from a wholesaler, the coffee is not free to the seller, it too is purchased from a wholesaler."
But as you say, there are infrastucture costs. That is what the whole issue is about. Even if the coffee and tires were free to the supplier, there would still be a cost for the store, lights etc. Stardock isn't charging for the skins. This has ben stressed time and time again. However, WC has an infrastructure (bandwidth, labor etc) cost that must be paid.
I like the library analogy even better.