I would buy locally in a minute if I could for only a 5% premium- hell, I'd pay 10%. But here in the great desert of Central Illinois, it's difficult to even find a dealer in reloading supplies, and when you do, or drive 50 miles to a gun show the powder is $22-25 a pound, etc. I buy mail order because I have to.
Buying locally is a great principle, but if people stuck by it, there'd be a lot more local hardware, appliance, and sporting goods stores. Instead, we have Wal-Mart, with its horrible business model, Lowe's, Home Depot, etc. If we're going to talk about patronizing local dealers as a rule, let's at least be uniform about it.
Think about it, and NOT just from the standpoint of shooting/reloading supplies: where do YOU shop?