[patmcc]

Follow your local laws. If wherever you live/operate requires you collect sales tax, collect it. Some places have weird rules about this; maybe you have to collect sales tax from everyone, maybe only from residents of your country, maybe only from residents of your state/province, whatever. Look into it. This is the one you definitely can't mess with, if you don't make the effort you can be on the hook for serious penalties.

Once you've done that, as greenyoda says it looks like you're clear in the US. If you have any other large markets it's probably worth looking into them.