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, although due to a recent court case in South Dakota, it's more likely that they will than before. States can force a merchant to collect state sales tax if the merchant has a significant enough economic presence in the state (either physical, such as stores, distribution centers, corporate offices OR online generally meaning $100k in sales or over 200 orders in that state). This isn't unique to online merchants--the same rules apply to any out-of-state company that you purchase goods from, which is why a car dealer in another state will still charge you the sales tax rate in your county of residence, rather than the location of the dealer.
Not too long ago, you could avoid sales tax with online purchases by doing a little research into what companies have no physical presence, but the South Dakota case often referred to as Wayfair, yes, that Wayfair, changed that so that most online merchants are collecting sales tax these days, even if they don't have a physical presence in your state.
It's important to understand that even if an online merchant doesn't collect sales tax from you, that 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. Often, states will provide an optional safe harbor “look-up table” to report a percentage of the taxpayer’s adjusted gross income as untaxed purchases in lieu of the actual amount of sales and use tax owed.