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1. Amazon issues tax documents to sellers each. Should he set up an LLC in US to sell on Amazon? Or should he use his personal name to sell on Amazon? Or should he use his Limited company's name to sell on Amazon? What are the pro and cons of each scenario?
Unless you select differently - for income
tax purposes - a single member LLC is a disregarded entity - means - all income and expenses are treated as received and paid by the owner. So in this case - the LLC is simply ignored. Still the LLC is a separate legal entity separated from the owner - it may own properties
(including real properties), provide some level of legal protection. The LLC may be liable for other taxes - for instance - sales
tax, import/export duty, etc. The LLC may have a bank account and its own tax ID
However - for income tax purposes - there is no difference.
2. Let's say he set up an LLC to sell on Amazon.com. Since he has no physical address is US, does it matter which state he sets up the LLC in? He heard that Wyoming is a good state to do online business for foreigners. Is it true?
Assuming he will not have any physical presence and will not use any facility in the US to fulfill his business operation - it doesn't matter in which state the LLC is registered.
There are several popular states to register the LLC - besides Wyoming - Delaware and Nevada.
3. And if he sets up an LLC in US, can his solely owned limited company in Hong Kong bill the LLC? That is, his LLC buys the cellphone accessories from his Limited Company in Hong Kong to sell in the US? Would that make accounting simpler since his limited company can bill the LLC once a year.
Yes - that is possible. But he would need to keep two books - for the Hong Kong LLC and for the US LLC. So I am not sure that would be simpler.
4. Also, what tax documents will he need to file taxes in US?
The US LLC is registered with state officials. He would need to file an annual report and pay annual fees to keel the LLC in good standing.
He might want to request the tax ID from the IRS
Only if the LLC had income from US sources - he will need to file individual tax return
as a nonresident reporting