Your routing needs to be set up along the lines of {controller}/{action}/{firstItem}. If you left the routing as the default {controller}/{action}/{id} in your global.asax.cs file, then you will need to pass in id.
routes.MapRoute(
"Inventory",
"Inventory/{action}/{firstItem}",
new { controller = "Inventory", action = "ListAll", firstItem = "" }
);
… or something close to that.