I guess tires are less complicated to install than parts that go under the hood. I guess it is like going into a restaurant and saying, I want that sandwich, but that bread you use is inferior. Please allow me to recommend another bread wholesaler.
If the tires you wanted to order online were the same tires he carries in stock, he would probably offer to lower his price and install his tires. Either that, or he might be inclined to refuse the sale.