If you are buying a lot within the EU, you don't need to pay customs fees when sending the order from one EU member state to another.
EU customs rules and regulations determine that if, upon import into the EU, the import duties and taxes are paid according to the transaction value of the object imported (which is likely to be the case for sales on Catawiki), the importer should be able to show the Customs Authority a commercial invoice prepared by the seller. Every EU member state has its own interpretation of which information that document should contain (it is not necessarily the same as a VAT 'invoice').
General information can be found here.
The most important information to have on this 'commercial invoice' is:
- The name of the seller and the buyer
- Description/size/weight of the object(s)
- Amount of objects, if more than 1
- Mode of transport and the value