eBay never sees the items concerned in disputes and unfortunately for sellers it's policy is evidently to take the buyers word against the seller's.
However, from the number of new, mainly cosmetic items you have sold you seem to have overlooked yhat buying stock to resell on eBay is a business activity. Private sellers are not allowed to do this.
You say yourself that you have a business to protec, and eBay requires business sellers to be registered as such. They are also required by law to accept returns. So in fact your buyer should have been entitled to return this item anyway, although because of your unlawful no returns policy they invented a reason to use eBay's money back guarantee instead.
So if it's any consolation, all you're really out of pocket for is the cost of return postage, which the buyer would have been required to pay if you had correctly accepted returns.
You can read eBay's guidance on selling as a business here: https://www.ebay.co.uk/help/policies/selling-policies/selling-practices-policy/business-seller-polic...
