This happened with CA (with NV gun shows and AR15 rifles/mags) but it's pointless in new england, as there are at least two entire states filled with dealers (excepting the like 3 box stores) where you can walk into the store and walk out with a bag containing magazines of your choosing. No need to even go to a gun show.
Not to mention the justification for a search is a lot more difficult then say what they used to do with the fireworks. If they were doing these kind of things we could literally set up Comm2A to f*** the state directly up the ass. It's going to be pretty funny when the state gets whacked on a USC 1983 suit or similar over a clearly illegal search, because the guy bought a bag filled with cleaning patches from a Maine gun store.

At least when MSP used to do the fireworks bullshit they could have the argument that /nearly everything/ sold in the store was illegal to bring into Massachusetts and that's not the case with a gun shop.
Even in the CA cases.... they literally had to track the person through the show, wach what they bought, watch them put the gun in the car, then rat them to an intercept car.
Also having experienced these people before, they're not great at being undercover. Work in an NH or ME shop for a year or three. A not insignificant portion of your customer base is going to be hardcore rkba, retired MA LE types. They would rat that motherf***er out to the staff in a nanosecond.