Since each state in the U.S. charges a different amount of tax, he's asking how much tax to charge since the website isn't considered to be in any state.

The simple answer is that you don't charge tax. This isn't an "essential" service, thus not taxable. If you make above a certain amount of profit within a year, you'll have to pay federal tax (call a government office and ask what that amount is for your state). So in other words, don't charge your customers tax if you sell non-essentials like pixel advertisements.

Also, I'd double-check with your state governmet to be sure. Get the number from 411 or Google it, and give them a call.