I'm buying a laptop online--will I have to pay sales tax?
There are currently five states that do not have a sales tax: Alaska, Delaware, Montana, New Hampshire, and Oregon. If you live in one of the other 45 states, or in the District of Columbia, you're probably legally responsible for paying some form of sales or use tax on your laptop purchase. And, to complicate matters even further, many municipalities assess a sales tax as well.
That doesn't mean an online merchant will collect the tax on your purchase from you, though. States can only force a merchant to collect state sales tax if the merchant has a significant enough physical presence in the state (think stores, distribution centers, corporate offices). This isn't unique to online merchants--the same rules apply to any out-of-state company that you purchase goods from.
So, it's entirely possible that you could order a laptop online from Company A, a company with an in-state sales staff, and have to pay sales tax as part of the purchase, but order the same laptop online from Company B, a company with no in-state connection, and not have to pay sales tax as part of the transaction. Of course, a merchant can voluntarily collect sales tax for a state, even if they're not required to do so.
Just because the merchant doesn't collect sales tax doesn't necessarily mean that you're off the hook, however. Most states that impose a sales tax also have a related use tax. Essentially, a use tax means that if you should have paid sales tax on a purchase, but didn't because the merchant wasn't required to collect the tax, you're responsible for reporting the purchase yourself and paying the appropriate amount of tax.
The details vary from state to state--some states include the use tax calculation on state income tax returns, while others use separate forms. Of course, because the use tax relies on individuals self-reporting their purchases, and states have a limited ability to enforce compliance, it probably isn't surprising that many consumers simply do not report their online purchases.