The sales tax is "fair" only in that everyone pays the same amount - but that's not really the definition of fair. Fair is when everybody pays the same percentage, because different people make different amounts. Let me illustrate this to you.
Guy A, a single male in his early 30's living in Tennessee, makes $150,000 a year. Guy B, also a single male in his early 30's living in Tennessee, makes $30,000 a year. They're both fiscally responsible, and don't spend outside their means. Here's the thing. Guy A is making 5 times as much money as Guy B. But he's not consuming 5 times as much "stuff" - basic, inelastic consumables such as toiletries, groceries, electronics, gas, and other non-luxury products that are responsible for the lion's share of sales tax revenue, because it simply isn't possible. There's only so much toilet paper one guy can use, and while his higher income may allow for the purchase of additional elastic luxury goods like cars, real estate, etc, the fact remains that Guy A is still being taxed on a significantly lower percentage of his income than Guy B, even if they're spending the exact same amount on consumables, say, $15000 a year. Even if Guy A's higher income leads to slightly increased consumption or investment (entirely plausible), leading to luxury tax revenue or capital gains tax revenue, it still won't be 5 times as much consumption or investment as Guy B. It's not fair in any way for Guy A to only be taxed on 10% of his income when Guy B is getting taxed on 50% of it.
So, in conclusion, it is better for everyone to pay at least something than it is for them to pay nothing, but everyone paying the same share is not the same as everyone paying their fair share.