The usual objection to a sales tax is that it tends to be regressive – the less income you earn, the more of it you tend to consume and the less of it you tend to save, so more of it gets taxed. The less income you earn, the higher the effective rate of your tax.
You can tackle this in two ways.
The first is to have a graduated tax, with low or no sales tax on which the poor tend to spend more of their income, and higher taxes on the things that the rich tend to spend more on.
This can work, but it requires constant fine-tuning. It also presents all kinds of opportunities for arbitrage, and distorts consumption choices. (If I buy bread, butter and cheese and make my own lunch, I pay no sales tax, but if I buy a prepared sandwich I pay sales tax. That is obviously going to affect people’s behaviour, with consequences for the prepared sandwich industry. That may be a fairly trivial disortion, but multiply it by lots and lots of other examples).
A second problem is that there is a constant temptation to set tax rates by reference to moral preferences or ethical principles, and this can work against setting them with the objective of making the tax progressive. Two examples:
Low earners tend to spend a greater proportion of their income on tobacco than high earners do. If the sales tax is to be regressive, therefore, tobacco ought to be taxed at a low rate or not at all. That is certainly not the current policy.
Conversely, the rich (who have health insurance) tend to a higher proportion of their income, directly or indirectly, on healthcare than the poor do, so healthcare ought to be heavily taxed. But, politically, that’s not really a flyer, is it? Same goes for education, I suspect.
While you can combine the two approaches, having no tax on healthcare and education and high taxes on tobacco, but regulationg taxes on other goods and services with a view to making the tax regressive, you’ll now have to manipulate the other taxes to a much greater extent in order to offset the regressive tendencies of the choices you have made with respect to tobacco, health and education. And where do you draw the line when it comes to setting tax rates on ethical grounds? If you concede tobacco, health and education, what is your basis for refusing to take a similar stance in relation to other major areas of expenditure?
The other way of making the tax regressive (already discussed here) is to have flat rate tax plus a rebate – every month you get a cheque which represents a refund of the sales tax on, say, the first $500 of your expenditure. If you actually spent less than $500, you’re quids in. If you spent very little more than $500, your average rate of sales tax, net of the rebate, is still very low. While this makes sales tax progressive as respects expenditure (the more you spend, the greater your average rate of sales tax, at every level), it doesn’t necessarily do that as to income. At the lower end of the range of incomes, where people generally spend the bulk of their income, the sales tax will be progressive, but as you move up the range of income, the more of your income you will tend to save, and the sales tax becomes regressive again.
The sales tax also penalises those who spend more than they earn (i.e those who are spending capital, including many of the retired, the more fortunate of the disabled and those who inherit capital and spend it). And, as has been pointed out, it’s particularly unfair to those who accumulated their capital out of income which was, at the time, subject to income tax, or who inherit that capital. It penalises those who have saved, in other words.
I don’t know if there’s any developed country which has entirely replaced income tax with consumption tax. Most operate a blend of the two, if for no other reason that a diversity of revenue bases is stronger and more flexible than a single revenue base.