I have reservation of them selling it on to some other wary customer.

The way to warn future buyers is to leave appropriate feedback about the condition of the Hoover when you received it.  But don't get carried away, keep it short and factual - long comments often end up including something that's not allowed to be said, when the seller can ask eBay to remove it.

As has been said, to get a full refund (with the seller also being responsible for the return postage) you need to use eBay's money back guarantee, see: https://pages.ebay.co.uk/ebay-money-back-guarantee/

As for the next buyer, it's up to them - just as it is up to you - to use eBay's procedures to claim a refund. Also to be aware that eBay actually warns buyers in the user agreement that they don't guarantee the quality of items sold here, that they will be truthfully described, safe or even legal. Would you have bought from a shop that stated this? eBay is a jungle.