The authorities responsible for collection of import duty and indirect taxes (VAT, GST etc.) will only deal with parties known to them, and where there are mechanisms in place for pursuing money owed, investigating possible fraud and so on.
If the seller already has a subsidiary or related company that is established in the importing country, then fiscal representation is no problem.
Otherwise the seller will need to find a party willing to provide fiscal representation on their behalf. This could be a freight forwarder, a customs agent or a logistics provider.
For the UK, the overseas seller is defined as a Non Established Taxable Person. The NETP appoints a fiscal representative who is registered with the UK authorities, and who conducts import clearance and tax/duty payments on their behalf
Many countries in the EU also require fiscal representation for the purpose of duty and/or VAT payments.
In practice, finding a fiscal representative can prove very difficult.
The role may require significant administrative effort, and will also involve exposure to financial, regulatory and operational risk. So typically these arrangements will require sellers to provide guarantees for reimbursement of payments made on their behalf.
(NB for UK to EU sales, the EU-UK Trade and Cooperation Agreement allows for waiving of requirements for fiscal representation under some circumstances. The situation here is fluid, and subject to frequent change)