I feel for Mike. Even if he ultimately prevails it's going to cost him in time and money.
I'm really wondering why BATFE is involved in this. Selling off-list guns is a state issue. True, an FFL dealer is required to comply with state and local law, but in the past, this has been handled with a nasty letter to the dealer from Martha, and to the best of my knowledge, it never involved the Feds. (I'm thinking back to when Bass Pro first opened.)
FFL's are already required to keep meticulous records. Contacting the buyers of certain guns and asking them to confirm the fact that they made that purchase sounds to me like they're looking to corroborate the information that has already been entered in the dealer's records. The only reason that I can think of for doing that is to gain more leverage to get Mike to plead out. "Mr. Wheeler, we have verbal confirmation from x people that you sold guns that you shouldn't have. If we go to trial, we will subpoena those buyers for their testimony in court."
I could see the state doing that, but why would BATFE be doing the questioning? Is BATFE now doing the legwork for the state?