I live in Switzerland (none EU) and do some Ebay shopping in the UK. Today for the third time I receive a package and have to pay import duty due to the fact that the seller fills the customs form incorrectly. The sellers put a higher value on the form than I actually paid. In Switzerland we're free to import (legal) goods up to a value of 60 CHF (= approx. £ 30,-) through the post. I always keep this in mind when shoppping on Ebay or any other UK website. It's therefor very frustrating to receive a bill from the customs authority for almost £ 15,- when the actual goods cost me only 4.50 including p&p.
My advise to international sellers is: fill the green customs declaration form accurately with an EXACT description of the goods and the actual value of the shipment (ie the price the buyer paid).