If the item meets some required conditions, a 404 is returned. The right code would be 409, conflict.
I am not sure, I thought about that at the time. The waiting list is (currently) an abstract concept as it consists in a list of items with a given status. But we could image it to be a separate DB table at some point. In that case would not we want to return 404?
Ok, I get it. Hope we can implement it that way soon!.