I presume the seller's returns policy stated in the listing was that they did not accept returns? If so, I'm afraid you accepted this when bidding.

Despite the steps that eBay has taken with HMRC there is a continuing problem of businesses passing themselves off as private sellers on eBay. There isn't even a specific option to report sellers as unregistered businesses.

This isn't surprising, as eBay is not a law enforcing authority. It can only enforce its own  terms and conditions - and these don't give buyers a right to cancel. The law does, when buying onine from UK business sellers, but enforcement of the law is a matter for statutory enforcing authorities.

As always, if you wish to buy in a properly regulated environment, consider this before buying from unknown sellers on eBay. The user agreement specifically warns buyers that eBay doesn't guarantee the quality of items sold here, or that they will be safe, legal or truthfully described. So this defines the level of trust you can place in many eBay sellers! Buying here isn't like buying from the shops. eBay is a jungle.