Whatever about the VAT, the first part of that statement still stands. Failure to display the euro sale price of goods is illegal. I doubt the traders travelling from the UK are mere hobbyists... the least they could do is show their potential customers some respect by taking the guesswork out of the equation and displaying the cost of their wares in the local currency. It's not a lot to ask, in fairness, and it's the only real downside to what is a very well organised event.*
*Edit: And I should point out, the issue is with the individual traders and not the organisers.