And if a contractor, being a good businessman, can save on labor costs (time) by buying everything at Home Depot and pass the costs of materials directly to his customer (plus his markup), what's his incentive to have to go to 3 or 4, or more, different businesses to buy various things for his job?

We need to get back to people expecting to do business with local business and give up a little in efficiencies. We're just making it easier for the Marxists/socialists to consolidate everything and starve out those who don't support their "workers utopia" when they get complete control