If you’re based outside the US but selling to customers within the US, you may still be required to collect and remit US sales tax depending on your "nexus" or business connection to specific states.
No US presence and no US sales?
If you don’t have a physical presence in the US and aren’t making any sales to US customers, then you don’t need to worry about US sales tax.
Selling into the US but based internationally?
You may still have an economic nexus. Economic nexus means that if your sales into a US state exceed a certain threshold (either a dollar amount or number of transactions), you’re required to register, collect, and remit sales tax in that state even if you’re located abroad.
About half of the US states currently have economic nexus laws. You can find detailed state-by-state thresholds here.
Living in the US, but not a citizen?
If you reside in the US and run your business here, you likely have sales tax nexus in your home state, regardless of your citizenship status. That means you must comply with local sales tax laws, just like any US-based seller.