How Sales is taxed?

Thank you for the urls of some articles. My question remains, however. In the articles above this note, there is conflicting saying on the one hand that internet sales are taxable, and on the other they are not. But, for my purposes, I am not a reseller or initial seller. I am a private person who buys something online for personal use. I have not been taxed by any company that does not have a physical retail location within my state, such as Target, or Sears, etc. BUT I have come upon a company that does not have a retail or any other physical presence in the state of Illinois, where I am, but it wants me, its customer, to pay tax. The company website says its corporate headquarters is in California, but it is actually in Canada. It sells internationally to stores, individuals, and likely other companies. I believe it does have physical locations within some states, but not Illinois. Can it legally charge tax to an individual located in a state in which the company has not physical presence, including no sales representatives?