If you have income which is generated within the United State, you may be liable for income tax. There are 3 questions you need to consider to estimate if you will be liable for income tax or not:
1. Do you have any owner who is a USA citizen or USA residents?
2. Do you have U.S. employees?
3. Do you have a warehouse or shipping facility in the US.?
If you answered NO to all 3 questions : you will be tax free in the USA !
If you answer yes to any one of this above questions : On income you earn physically in the USA or on income from USA assets (USA stocks / real estate ) you pay taxes