That someone is wrong. You made an offer and they did not accept that offer. They can't construe the offer as they see fit (the money back) and make that somehow binding
As another issue, generally items are sold "as is." So unless there are other facts to the contrary (some aspect of fraud or a representation), the fact that it may or may not work for the seller does not matter. Now, as always, they certainly can try to sue in small claims and the judge may or may not find in their favor. Going forward, you are best off using a written agreement that makes clear that the item is sold as is for your records. Good luck.
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Agree with Mr. Murello. Your offer was conditional on the item not working, but you demonstrated it was working when sold. If there was no return policy when sold (as on eBay or Craigslist) or just a casual sale with no documents, they're stuck with it and you don't have to offer any refund. Save the text messages on your phone or any other documents (print out of craigslist/eBay listing) in case they bring a small claims action. Good luck!
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As written, your offer was conditional on the item not working. If the item works then you do not have to take it back.
If you'd like to discuss, please feel free to call. Jeff Gold Gold, Benes, LLP 1854 Bellmore Ave Bellmore, NY 11710 Telephone -516.512.6333 Email - Jgold@goldbenes.com