I'm surprised there is a bid on that domain,especially this early after the launch of .EU.The bidder is almost certainly theracingworld.co.uk,a kind of big site.
They are probably your one and only chance for an end user IMO.$500 is a great price for a name registered 3 days ago.There's literally no way to determine value when there's no past .EU sales...it's up to you.Your sale can help start shaping the market.
Do you take a quick $500 and reg some more .EU or hold out for more,counter offer,or potentially lose the sale? An accurate appraisal is literally,truly,impossible.