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.
The "sticker shock" is the same on the same thing priced the same. There is no price advantage or disadvantage caused by the FairTax.
None.