I would argue that buying local is sustainable only in a small and medium villages. the stock is small and service is excellent. I remember living in a small town overseas, the fish monger would come everyday in his bike to make custom cuts to fishes. he would always remember our families names and types of fish we buy. can't really do that in costco or a large city format.
sometimes its really sad to see how modernization has really "cookie cut" our society and the majority of our society loves it. there is a recent article somewhere about how a town in India has threaten the support of a politician if he was going forward in building a Walmart. have to dig it up.