This is spawned from @Dana Shultz's answer to this question.
from this portion of his answer:
...if you incorporate elsewhere but doIt sounds like I will have to register in any state that I do business? This could potentially be in all 50 states or out of the country. Do I really need to register in every potential place?
business in CA, you will need to
register in CA, too...
No. The key to Dana's statement is "do business in ". Every state has their own definition of what "doing business" in that state means, but in most cases selling a product over the internet to someone who happens to live in that state does not count as doing business in that state. You usually need something more tangible to be considered doing business in a particular state.
In your case, if all you do is sell a product on your website, then you are only doing business in your home state (because that's where you are working from). Therefore, you only need to register your company in your home state.
The answer above is right. Each state has a statute that defines what "doing business" in the state is. In general, anywhere you have a facility or an employee will require registration.