If you are resident in UK and below the VAT threshold then you do not need to be VAT registered. However, if you are buying (or making) goods to sell on for profit then you MUST, by law, be registered with HMRC as self-employed, probably sole-trader if it is just you. Can be done online. They will send you your UTR and you will need to change the legal entity of your account on Amazon - presumably you signed up for an Individual Seller Account. ?

You need to keep accounts and submit a tax return every year.

You do not need to have a ltd company - that would likely be further down the line, once you understand all the implications of a ltd !.

For invoices for business customers you can declare your VAT exempt status and Amazon will do it automatically for you, though any received before you declare will need to be done manually. Easy to create a receipt (note not an invoice), and make no mention of VAT.