When one asks "is taxation theft?" it presupposes that there is a state in existence to collect the tax, so within the realm of this presupposition you have to pick your poison...and in this case, the answer would be that sales tax is more consensual and thus, not theft.
Even in the case of an AnCap society that has privately built roads there has to be some means of collecting to pay for their construction and maintenance right? tolls, you say...right? but that's not a 'tax' right? you can choose NOT to use that road the same way that you can choose to not pay a sales tax, unless of course there is a means of enforcement.
So in the case of the AnCap toll road, if somebody decides NOT to pay the toll, who then has the right to deprive that person of their life or liberty for traveling on it without payment and wouldn't that entity have the same role to play as the IRS? There are many scenarios you can imagine where this could be the case. A medical emergency, for example.
Interestingly on a side note, if you think about it, all crimes are a form of theft. killing is theft of life. Lying is stealing the truth. Adultery is theft of one's trust and so on...
So what do you think?