Bull. Flat out bull.
People don't pick Amazon or Overstock to save on sales tax... they do it because the prices are cheaper.
Not always true. Stores like Fry's match online prices. People pick Amazon because it's more convenient, there's a wider choice of products, or because they don't (didn't) have to pay sales tax.
You picked out cables as an example. Best Buy is notorious for selling absurdly overpriced "premium" cables with gold-plated connectors or some other stupid gimmick that only idiots would spend money on. But if you pick identical products, there's not that much of a price difference.