Hi fewleh,
The commission charged by Etsy was correctly charged due to to Etsy believing that you were not in business, hence the charge of VAT.
As you stated the VAT should have been subject to the reverse charge.
I would recommend contacting the marketplace and confirm that you were VAT registered during the time they were charging you VAT on the commission.
Although they are not obliged to amend the invoice they can amend this and subject the invoice to a reverse charge transaction which you can then use to submit your VAT return.
However if the marketplace refuse to amend the invoice then you would not be able to recover the VAT as input tax as it has been incorrectly charged to you.
Please see the guidance below
Place of supply of services (VAT Notice 741A)
Thank you.