I agree it very much sounds like this buyer is being dishonest and just wants to get back the $100 profit.   The 'repair quote' exactly $100??  I don't think so, too much of a coincidence. Where is the evidence of a scratch??  No proof?  Not surprising.   If there was one you would notice when you unwrap the camera from the packaging and that's when you would complain.  Not after a month's use which is obviously what has happened.

Fight this dispute.    I hate seeing dishonest buyers take advantage of sellers like this.