Of Arms and the Law writes that the BATFE is looking into the "stings" that form the basis of this lawsuit. This newspaper article has more details on exactly how those stings work; it seems they sent two "investigators", one a retired cop, trying to buy a gun without the right paperwork, then sent them back the next day with the right paperwork but two numbers transposed when the dealer refused to sell, and then tried to talk the dealer into completing the sale with the wrong numbers. The dealer called into the office that issued the paperwork and got permission to update the numbers and sell the gun. End result: no crime committed, but Bloomberg gets a very public raid.
Later, of course, the dealer gets a very private "Here are all your guns back, sorry," and a warning of "heightened scrutiny". Typical. This is about intimidation more than anything else.