Hi,
It is important to establish where you are a non established business or not, as this does have a significant effect on the VAT treatment and whether you would need to register for VAT.
Who should register for VAT (VAT Notice 700/1)
If you are non established in the UK and the goods are held in the UK when sold, then it is the responsibility of the marketplace to account for the VAT. You would not need to register for VAT but you can do so voluntarily to recover import VAT that you incur while making a deemed zero rated supply to the marketplace.
Changes to VAT treatment of overseas goods sold to customers from 1 January 2021
If you are established in the UK then it is your responsibilty to account for the VAT. Please look at the guidance carefully and speak to the marketplace so that the VAT can be accounted for by the correct party.
If you need to change your address with HMRC, you can find guidance here:
Change your VAT registration details
Thank you.