I have been eBaying for 30 years now, having 6000 100% positive feedbacks.
However, I don't eBay much these days as it's full of scammers and opportunists.
Your major error was this
'The buyer then submitted an 'item is not as described' refund request which I accepted.'
You allowed the buyer to manipulate the eBay system in his favour. Item not as described gives you absolutely no protection from eBay. It was as described, so you should not have accepted that reason as the cause for return. Doing this, you shot yourself in the foot, and now you don't have a leg to stand on. Ebay frowns upon dodgy sellers. INAD is a black mark against you as a seller. You should have rejected this and asked the seller to correct the request, so you could accept the return. That the item is as described, so you could not refund for that reason. Better still, phone eBay before agreeing to anything, and explain to them.
It's a hard way to learn a lesson. If you are going to eBay you have to protect yourself by putting YOUR OWN Terms and Conditions on EVERY LISTING.
These include what the postage charges consist of. The definition of the condition of the items, i.e used means the item has been worn and is not in NEW condition. There may be minor signs of use, or if there are signs of use, show them in the pictures. Also, state the pictures form part of the item description and should be inspected thoroughly before bidding. You can also specify NO BIDDERS with LESS THAN 50 Positive feedbacks. This helps and you can remove bids and refuse to sell to new bidders who are potential scammers. Your T&Cs protect you.
By bidding on my item, you are fully accepting my T&Cs and no refunds will be given unless the item is grossly not as described.
Sorry you had to go through this. Hopefully, you didn't leave feedback for the item and can leave actual factual feedback. The item was brand new and tagged. Buyer returned item dirty, stinking of smoke, obviously worn, and leave 1 star and negative. This at least warns others of their scam.
I no longer leave feedback until 4-6 weeks after an item is sold, or until after feedback has been left for me. People sometimes prey on new eBayers because they know they are easy targets and too trusting.
Sorry, it's a waffle but I hope it helps.