I don't see sales tax as theft. Added cost? Sure. But then again, so is paying 10c for a paper bag at Aldi. Don't like it, don't buy that. No one shows up and arrests you. You have a choice. Income tax, you have no choice.
The decider for me, is choice.
Except that there is no choice, because the government requires the seller to collect sales tax as a percentage of every item sold and remit it to them. So that seller can either pass that cost on to his customer, or he can eat that cost out of his own profits.
It's true that the buyer can choose not to buy, and the seller can choose not to sell, but in a capitalist system, that is what we do. Unless we all choose to become individually self sufficient and produce all the products we need to survive, we will have to participate in some form of exchange. That is the result of the division of labor. Under our current societal structure, we can't all be cops, or goat farmers, or architects, so we trade for the products we need through a form of intermediate exchange.
And every time we sell products, we are forced to pay taxes on those sales to the government. It is natural and understandable that the cost of those sales taxes would be passed on to the customer. Some politicians are proposing service taxes now as well. Read here, here, and here.
So yes, sales taxes are theoretically voluntary, but in practice, they are unavoidable. There really is no choice at all. After all, the seller could just choose not to remit those taxes, and eventually the guys with the guns would show up to explain to him the error of his ways.
-Rob