Customs documentation will be required in order to import the goods into the EU. Whether the buyer or the seller will be responsible for this will depend on the Incoterms relating to the transaction. For example if the goods have been sold on conventional CFR terms or ex works then the buyer will be responsible for the importation into the EU, but if the incoterm is DDP then the seller is responsible for the importation and the documentation. https://www.gov.uk/check-duties-customs-exporting
More complex transactions than a third party sale (eg movement of own goods) means import procedures into EU as well as UK export need to be considered, and an EU VAT registration may be required.
The information required for customs documentation includes an EORI for UK business, commodity code for the goods and origin certification.
In certain sectors EU non tariff barriers may need to be considered eg food SPS etc
In the absence of a comprehensive free trade agreement there may be customs duties payable on the goods when they arrive in the EU – although the customer may be responsible for paying them, they may impact on the price which has been agreed for the goods and contractual agreements may need to be revisited. The current EU tariff can be found at this link: https://madb.europa.eu/madb/euTariffs.htm