They don't change their prices based on where they have to charge sales tax. Price also really not much bearing on the need to collect sales tax. The point that many try to make is that customers will be turned off when they have to start paying tax for their purchases and it will bring prices more in parity
Of course they change their prices based on where they have to charge sales tax. The price becomes the price + tax. Even post-tax, Amazon will usually beat Best Buy.