Say I'm selling at a swap-meet.
A person who, unbeknownst to me, is operating another store in the same area approaches my stall and purchases an item (specifically an old, but still functional laptop), then later demands a refund after removing the screen and hard drive claiming "you said this still worked, but there is no screen or drive." Shortly after providing a picture taken before opening (featuring the device still intact) and then pointing to a plainly-written sign stating "No Refunds," the person proceeds to make a loud fuss in front of my stall for an hour or so.
As another example: Say I'm selling something on ebay.
Another store buys a $100 item and mutilates it upon receipt, then demands a refund claiming "not as described." I refuse the refund and receive a negative review.
I'm tempted to call the behavior in either case "fraud & defamation of character," but I'm not sure if that applies here...