A few thoughts:
A crack in a frame can exist for a long time without being noticed. The number of safely completed rides, though, are no indication of the cracked frame's potential for further safely completed rides - the frame must be retired after the crack is discovered because cracks never get less serious with time and use, only more serious, and will eventually result in a more serious failure.
If the buyer is requesting money refunded and bike returned to seller, this is in no way a 'scam'. All monies and property will be in the same hands they were ten days ago, as if the original deal never happened. Unless there is evidence of the buyer crashing or riding abusively then it is reasonable to assume the crack was there but not noticed when it was sold.
I had about ten thousand kms on my first touring bike when I noticed the crack. I had noticed creaking and unusual 'squirming' in the rear end for some time before that. I suspect the frame had been cracked for months and probably hundreds of kms before I finally found it (crack was in the drive side chainstay, hidden behind the chainrings).