Amazon is a great place to buy things! Amazon sell things on there and so do independant sellers.
If you really want to buy directly from amazon then find your product and click on it and it will give a list of the sellers and one should say amazon.
No you cannot use amazon if you are 16 but it’s fine if you get your parents to buy things for you : ) No you cannot use giftcards unless you have a special amazon giftcard that i’m not aware exists : ) Amazon products are not taxed but you will have to pay a small postage fee : )