Australian wanting to start an online business in the US
You may register a business in the U.S. without being required to physically be present. You are not required to be a resident or citizen of the U.S., and you do not need to have a work permit, visa or other documentation to register the business. Typically, we recommend forming an LLC as they are a more flexible and easier to maintain. You can click here to compare the differences between LLC vs. Corporation.
With an online business, you are technically free to form a business in any state. That being said, it may be beneficial to consider registering your business in Wyoming or Delaware, as they are considered the 'corporate havens' for their business friendly atmospheres.
Keep in mind, if you are going to be using your home address in Washington as the business address, and plan on writing-off your business expenses, then it may be the best idea to initially register your business in Washington state.
Sales tax will be collected on behalf of the state your business is registered in or has nexus. Keep in mind, if you will only be selling info products you might not even need to collect any sales tax on those sales. The sales tax would apply only to tangible products.
For example, Delaware state has no state sales tax, so by registering a business there, you are not responsible for collecting sales tax from your Delaware customers to forward those taxes to the state, but if you will ship the products from Washington state you would need to register and collect sales tax from orders shipped to WA state. And of course, you form the business in Washington, you would need to charge your WA-based customers the WA state sales tax, and remit those taxes to the state.