The local guy can't keep his doors open by just mounting tires people buy on line. He makes $$ on the tires too. If he's off by a few dollars, give him the business. If he's off by a lot, tell him and see if he'll adjust his price.
I guess I'm just saying to look at the *bigger* picture. If he's close, give him the benefit of your business. If he won't move and he's way off, then do what you have to do....
Just my $.02