15% added to anything $100 or more is going to drive me to Craigslist both for buying and, knowing that others will check secondhand non-regulated exchanges more, selling.
Secondhand non-regulated exchanges are already players that retailers now must compete with, since such exchanges, thanks to the Internet, are ubiquitous. Add a large sales tax, and they become even larger. This was a problem the FT took seriously a decade ago, and such exchanges have only grown since then.
Further, imagine I wanted to move across town. Say I want to move from a relatively cheap house of a certain size to a nicer house of the same size. This is something persons do. So, I decide to move out of my $200,000 house for a $250,000 house. Except now I have to pay $45,000 extra? Why would anyone in their right mind make that move, when for $95,000 they could turn their current house into something better than the $250,000 house? Are you now going to tax services as well (something not done by the FT)? And, why wouldn't I then just offer subcontractors money under the table to do the work for less than I would pay in taxes, afterall, the is no IRS and no reportable income, right?
Same with a car. Say I want a nicer car, but now buying a new $40,000 car will cost an extra $6,000. Why not just spend money at a shop, money that mostly goes toward labor, and, again, pay under the table?
Sure, some might irrationally just want to pay exorbitantly higher prices just for something new, but why would I grant that as the norm and give you that as a low-ball figure?