Contact your local tax office and ask, or look through the terms and conditions of thr online outlet to see how they deal with tax (the last thing you want to be doing is paying more than you actually owe).

Not sure how it works in the US, but in the UK if you're selling commercially (which you are) then you will have to register your governments tax system (UKs is HMRC), which im pretty sure you could probably do online.

Where you register should have way more helpful information than what anyone here can provide. Good luck