Put the numbers down for a moment. Why would the Seller care about the price of a product if he did not pay taxes. Just think about that because that is what you flat taxers are saying.
The seller worries about it because of price resistance or "sticker shock" and thus lower sales -- the buyer cares what he is paying out of his pocket, not what the base price was. The FT fans claim that lower prices are advantageous, which conversely means that there is a disadvantage to having a higher gross price, if you agree with them.