I understand your frustration and dilemma. I, personally, wouldn't take it back after that much time had passed. But I would have said so in the beginning. There, generally, are NO RETURNS in a venue like this. I even have a sign in my antique store that says NO RETURNS or REFUNDS. They have plenty of time to inspect/test the item before they purchase or leave. I had a woman pull the same thing on me with a very expensive candelabra. Dresden Porcelain, she sepnt an HOUR looking it over and talking about it with me. Handled it and haggled on a price with me. When we were both satisfied she bought it, and them had the NERVE to phone me two hours later to tell me she discovered damage. I told he no returns, refunds, or exchanges. Sign was posted, she inspected, I pointed out damage/flaws to her, that she bought it KNOWING it was damaged. I stuck to it too.
We live and learn with every sale. I still learn lessons 6 yrs later selling/shipping on Ecommerce sites. I had one lady get mad at me because she came to pick up an item (through OfferUp) she decided since I was a STORE she'd use her credit card instead and I charged her tax. Most people show up with CASH in hand or use PayPal.