Thanks, more questions

Hi, 南陽山人, thank you very much for your reply.
Here are several more questions for you:

1): For what reasons, you think Delaware and Nevada are the best states to start up a company?

2): If I want to open a business in Nevada or Delaware, do i have to live in that state? Do i have to register my company in every state or just in the state where I live if I want to do my business nationally (such as sale in the internet).

3): I know many people doing business online whithout registration with Secretary of State. Is it ok to do this way if people pay the tax over the income they received? For example, many people sell thier stuff in eBay and IRS consider the money taxable in most cases. so it is ok to do business online without registration if you pay the tax you should pay, right?
If so, does this rule only apply to US citizens or no restrictions?

Thank you in advance.