This happened to me once, but with an amp. Guy came to my house and played it through his bass with my cabs, purchased it with cash, and left. All was good. The next day he texted me that the mid control didn't work, and that he wanted to get his money back. I told him that I was unaware of such a defect, that he had checked it out prior to buying, and didn't see anything wrong at the time, so I refused to give him a refund. He was pissed, and told me so via text.
But I was confident that the amp was in working order prior to selling, and had zero clue what could have happened to it after the sale. No way I was going to take back a malfunctioning amp that some random person used for 12 hours when I knew it worked fine the day before.
It's the only time I've had an issue buying/selling in person or with cash.
Well.......except the time I sold a motorcycle to a guy that WRECKED THE BIKE ON HIS TEST DRIVE. Dude took off, was gone for a few mins, then came back with the brake lever broken off, turn signals, handle bars, and entire right side of the bike scratched due to him clearly laying it over on his ride. He said, "Man, this thing is way heavier than I'm used to. Not sure if I'll be able to handle this." I looked at him in utter disbelief. Several awkward moments passed in silence. Then I said, "You'll get used to it." in as firm a voice as possible while looking him directly in the eye. He took the hint, and was man enough to purchase the bike from me. Talk about a frightening moment!!!