Does anybody know how much sales tax is on the Xbox one marketplace?

It would be the same tax rate as the location you told it in your profile.

So if you were truthful and live in an area with 7% tax, then you'll be taxed 7%

Simple answer: Buying stuff online isn't subject to sales tax...unless you live some place where it is.

Complicated answer: If you live in a state where the website or the company has a physical presence (office, store, or even a warehouse) then you pay your state's normal sales tax. To make things more complicated, if your state allows each city to set its own sales tax, then you pay the sales tax of the city where the online company's office/store/warehouse is located. I have no idea what happens if the company is in multiple cities in your state and each city has a different tax rate but knowing the government, they probably just assume the highest tax rate.

If your state doesn't have sales tax, or the company doesn't have a physical presence in your state, then you don't pay sales tax.

It's 6% in my state.

Sales tax depends on your city. You can check what your rate is at http://www.sale-tax.com/ and then enter it into http://www.sale-tax.com/Calculator to get the total plus tax.

Microsoft charges taxes on games so a $20 gift card wont get it.

http://www.microsoftstore.com/store/msusa/en_US/Di...

That should depend on your location.