Sales tax is not like the VAT in Europe. It is a one time point of sale retail tax limited only to transactions between the retail seller and the end buyer (consumer). It is also not a national tax but a local tax controlled by the states and local governments. So there are literally hundreds if not thousands of different sales tax rates in the USA. Traditionally this complexity was lessened by the interstate sales tax exemption. All sales between different states were generally sales tax free. So if I bought an item from a company in California by telephone or mail order, and it then mailed the item to me in Texas I would not have to pay the sales tax in California or Texas. 

Online sales have significantly complicated things, but not as much as some think. Not all sales are covered by sales tax but often a company will choose to just collect and pay the sales tax to avoid any problems. After all if they sell an item for $10 and the tax is $0.80 the buyer pays the sales tax in addition to the base price; $10.80. Unlike the VAT the sales tax is not hidden within the base price.

Whether or not a company needs to collect the sales tax will be determined on a state by state basis but in general the measure used is the NEXUS approach. Does the company have a significant presence or contact within the state; is it chartered there, does it have a primary office there, does it maintain warehouses and transportation facilities there?

In the past this was fairly simple to sort out. You set up your company in a small population business friendly state like Delaware, Wyoming, Nevada, etc. You usually had a distribution center in that state or some other small population business friendly state. If you could not do that worst case scenario would be that you set up your company in your home state and pay sales taxes on sales in that state but then were exempt from sales in any other state as long as you had no significant presence in that state. So a company set up and operating in California or Texas would only pay sales tax on sales in its home state. This is still the case for most businesses.

With the introduction of Amazon and its Fulfillment by Amazon (FBA) program that has changed. If you sell through Amazon FBA then you are considered to have a NEXUS where ever your products are warehoused by Amazon. Amazon has dozens of warehouses across the country.

If you set up a company in Wyoming (a very small population and a very friendly business climate) and you have no other presence in the USA other than that, and you drop ship from outside the USA or just ship from a single location inside the USA, you will only have to pay sales tax on sales inside Wyoming and possibly the location of the warehouse where your products are stored. If you keep everything inside Wyoming, considering the tiny population of Wyoming that might be close to zero.

However, if you use Amazon FBA (or some other 3rd party sales distribution system like it located inside the USA) you will need to also register with every state where your products are being stored by Amazon. Amazon makes this information easily available to you. Last time I checked, the primary warehouses for Amazon are located in California and Texas. You will almost certainly need to get a sales tax permit in those 2 states in addition to whatever state you chartered your company. As your product becomes more popular in Amazon, Amazon will stock it in other warehouses in other states meaning you will then have to register for sales tax in those states as well.

There are a few companies that offer sales tax accounting services for companies using Amazon FBA and other 3rd party distribution services in the USA since it has become so difficult and costly for small companies to maintain the accounting staff to do it themselves.

Remember, if you are NOT using Amazon FBA or some similar service none of this may apply to you.

Just remember, the US sales tax is not the VAT. It is totally different, and if you think your understanding the VAT, with all its complexities, will help you understand the US sales tax system you are mistaken.