[saurik]

However, if you do business internationally, this is not as clear: you do not have any a priori right to take money from citizens of other countries for your products or services; as a specific example, if you have customers in the EU (and you meet the various thresholds in any of the member countries) you must register for VAT (probably through the VAT on e-Services program designed for streamlined registration of overseas companies selling digital products). Every quarter, for example, I have to remit collected VAT for every EU country through HMRC (in pounds sterling, which is really quite irritating).

(Even domestically, the rules regarding whether you are doing business in a state are dependent on the state. If you travel at all, have any contractors, work with any other companies, or even simply ship goods around, it can be very confusing to determine in what jurisdictions you suddenly are liable for collecting either sales or seller's use tax. I ran into this personally while trying to figure out "am I doing business in Michigan, a state that imposes a 6% tax that includes digital goods?": the Michigan Treasury didn't want to commit to an answer, so I was going back/forth with accountants and lawyers for quite a while.)