I have letters from both the dealer and the owner confirming duty and VAT had been paid. Under EASA, the change of state is now a non-issue - although going back through 30 years of French tech logs to check of AD's being complied with, took a bit of work. Babelfish is your friend...
However, in practical terms, unless you are buying something out of company ownership, within the EU it is pretty well bound to be duty paid. The red-rag-to-the-bull for the customs mob are aircraft owned within European corporate structures or on the November register. It's not right, but it is the way things are.