Firstly, if you send anything by standard mail without tracking then by default you do not have seller protection whether or not you are a business seller.
Next, if you state no returns in your listing this only applies to buyer remorse (change of mind). For all other return reasons you are bound by the eBay money back guarantee rules. When you signed up as a seller you would have read all the terms and conditions, I suspect you just forgot these couple.
Once a buyer claims item not as described then it's extremely difficult for you to argue against. EBay will take the word of the buyer.
So... If you want your item back you will have to pay for fully tracked postage. EBay requirement.
Your only other option is to fully refund and buyer keeps the item.
The only other slim chance you might have is to negotiate a partial refund but good luck with that path.
These are the rules of selling on eBay we all agree to. If we can't get along with those rules then your only option is to find another selling platform more favourable to you. Good luck with that too.
Others may come along with other suggestions.