For eBay to have charged you, this must have been a credit card chargeback or similar and the £14 is eBay's admin fee for this.
I don't have time to look up eBay's exact t&c re. chargebacks, but they are supposed to indemnify sellers against non-delivery claims if they provide proof of delivery, or possibly even proof of posting - I would have to check.
It would be difficult to comment anyway without knowing when the buyer received the item, i.e. before or after the dispute was opened, also we don't know how you responded to the dispute.
If the case is now closed, which I imagine it as if eBay has charged you, I doubt you will get the fee removed now. Good luck anyway.
To read the conditions for youself, see (It's near to the end of it): https://pages.ebay.co.uk/ebay-money-back-guarantee/