Any agent can submit an online EC Sales List on behalf of a client but the agent must be registered for VAT and appointed online by the client to act on their behalf.
The client should enrol online as an organisation for the EC Sales service through HMRC: New user. An Activation Code will be sent (by post) and once the client has activated the service, they should log in with their User ID and Password and select 'Manage services', then 'Appoint agent'. They should then enter the agents reference number which links their VAT number to the agents online account.
For further details please refer to How to use VAT online services for agents.