To me, that is bad manners, and that goes for both of you.
Import charges/taxes are not the seller's problem. Just deal with them (or arrange something with the seller beforehand).

To the seller: You can't have both the money and the module - are you insane?? Were is the logic in this? You got the module back - refund the guy and stop crying.
(Personally, I wouldn't refund the postage).

Yes, we should probably have a "dubious sellers/buyers thread".
-- KNYST

Thanks for all your replies on this. I agree with KNYST on this. It was bad manners on my part and I apologise to everyone here for that. I got an email from PayPal this morning to say that they have found in my favour and will get me a full refund.

What I've learned:

It costs WAY more than 20% tax to import something into the UK. It's actually over 30 after the fees.

Switzerland is NOT in the EU!

If there is even a question of import duties, I'll make sure we are clear about it before money changes hands. Not clear like "it's on you bro!" But clear like "it's likely to be over 60 quid, understand?"

Cheers guys