First of all, whether you are required to legally form and register your business in USA or Korea does not depend on your will when you do the online shopping mall. In general, there are two key tie breakers: 1) where is the online server located (USA vs Korea)? & 2) Where is the warehouse?
If the server & warehouse are located in Korea, generally you need to register your business in Korea and obtain a sales permit. If they are in USA, of course, legally form your business in USA.
Sounds like you are planning to sell US products to the consumers in Korea ONLY. In that case, if your business is domiciled in USA, you don't need to levy sales taxes.
Setting up a business in USA should follow the regulation in your domiciled State. First, you will have to decide which legal structure of your business is going to be... such as LLC and Corporation.
Because you are a US resident for the purpose of income taxes, regardless of your business location whether Korea or USA, you will have to report the business income to IRS and your domiciled State even if your business is established in Korea.
한국에서 간이과세자란, annual sales revenue가 원화로 4,800만원 이하일때 선택할수 있습니다. 일반과세자와 간이과세자는 부가가치 계산방법과 신고해야하는 의무가 다릅니다.
감사합니다.
김광호 공인회계사