This is such an over blown argument. Sure, Amazon is ~7% (where I live) cheaper than traditional stores due to sales tax. But that $1000 Wal-Mart laptop has a $900 sticker price on Amazon, and most non electronic items are 20-40% cheaper than in stores.
If laws change and I have to start paying sales tax on Amazon, it won't change a thing about my buying habits.
Edit: They also have an inventory many times larger than any brick and mortar store. Whenever I go shopping, I have to choose between the least crappy option Wal-Mart decides to stock. On Amazon, I get exactly the one I want.