If you paid, and you want a refund, you will have to return the item when it arrives.

You would have to pay for tracked postage, but that would still be less than £35.99.

You could also ask the seller to cancel your purchase - they may agree if the item hasn't been sent.

Unless the price of blu ray items has shot up for some reason it's possible that pricing it at £5.99 was a typo, which the seller corrected when they realised their mistake. It's a pity you didn't buy it as soon as you saw it!

If some sort of supply restriction is being exploited by sellers to charge artificially high prices this might be covered by eBay's policy on price gouging, but this seems unlikely. Price gouging doesn't apply where worldwide demand has raised prices univrsally - as in the memory upgrade I bookmarked for my PC a few months ago, but quadrupled in price withn weeks.