She will only have to collect tax if shipping to your home state(for companies if they have a presence in a state then they must collect sales tax on items delivered to that other state(s)
In the real world until she sold say 10k/yr she would not even have to file an income tax form. At that point she would be allowed to deduct normal business expenses. If this is just a hobby, and she is selling only a few items at a time, while legally she would need to file for a state tax certificate, obtain permits, or other legal obligations. So MY advise, (I'm not a CPA, just an CFP)is just sell these items on ebay, or perhaps wholesale to local merchants. She is not renting space for an assembly plant, warehouse or retail store. If, after HS(and hopefully College)she wanted to do this, and sold say $50k THEN she would need to be more formal, but she is not, so she doesn't even have to think about any formalities until she has a significant profit. The bureaucracies would have her running to get permits, licenses, and a hundred other things, all so they have work to show how valuable they are, lol.
For the most part you just have to report profits and pay income taxes, one might have to charge sales tax when selling to someone in the same state, one might have to register with the franchise tax board, or whatever it's called in the state.
Being under 18 is going to be an obstacle at every turn, since basically contracts with minors are not binding, probably needs a supportive adult to sign up on her behalf.
Etsy.com is the big hand crafted goods marketplace that's easier to start with than creating your own site,.
eBay doesn't report to IRS unless she sells 200 items or make over certain amount of money over eBay. I'm pretty sure it's the same way with PayPal.
She could try Etsy, but I think they report the sales.
That depends on what she is selling.
