Hi kjmr,
If the overseas company sell goods below a value of £135 and they use a marketplace then it is the responsibility of the marketplce to charge and account for the VAT over to HMRC.
If the goods are over £135 and the goods are sent directly from overseas then it would depend who the importer of the goods is.
If the customer is the importer of the goods then they will pay the VAT themselves.
If the overseas company is the importer then they will pay the import VAT and then make a domestic sale plus VAT to the customer.
If the goods are over £135 are already in the UK then it will still be the reposnsilbility of the marketplace account for the VAT on this.
Please see the guidance below:
Changes to VAT treatment of overseas goods sold to customers from 1 January 2021
Thank you.