I know of only one case, and that was similar in that a gun sold by the C&R FFL holder turned up in the wrong place and wrong hands. He got a pretty stiff warning that selling to non-collectors could result in revocation of the license, but that was all. That was in VA. In MD and some other states, had he not followed the law on individual sales, he could have faced a stiff fine or jail time.
AFAIK, BATFE does not conduct any routine or periodic audits of C&R licensees.