No real help, sorry (others posting here know more, are wiser and are more experienced), but may I offer a few thoughts possibly worth pondering:
Did you film the exterior and in the same filming sequence film yourself packaging up the item and taking it to the courier? Of course not, who does that?! Can you prove the mark wasn't made during shipping? eBay probably wouldn't be interested anyway. All of this paragraph is to show how non-watertight things are that people rely on e.g. pre-sale photos aren't proof (item can be damaged anywhere between photographing and drop-off), nor a courier's/Royal Mail's proof of posting (only proves an item of a certain weight was posted). Obviously there's a lot of trust involved in a transaction.
By stating "no returns" a private seller can't legally cancel a buyer's rights under consumer law nor eBay's MBG/policies (which tend to favour the buyer), even if a buyer allows a seller to do so in practice.
Did the buyer film themselves unpackaging the hob and film the exterior? (I do that when I buy high-value items). Doesn't sound like they did, yet it's reasonable to expect a buyer to check goods on arrival, before getting the hob installed (though not surprising, human nature being what it is).
To avoid having to accept a return and/or refund, maybe consider offering a partial refund? but without prejudice (you don't have to say "without prejudice" which makes it sound legal or officious, as long as you make it clear that you believe you sent the item without a mark and would be making the offer on the basis the mark may've appeared during transit or after and you thus can't be held responsible for the mark, and that if the buyer accepts your partial refund then that ends the matter), and always communicate using eBay's messaging, and write as if you expect eBay to read the messages.
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