I'm a LEO in NJ, so take it for what it's worth. High capacity magazines are legal for anyone to own in NJ as long as they are not capable of holding more than 15 rounds. The magazines can either be blocked with a piece of dowel on the inside, loading the mag with enough empty brass so it can't exced the 15 round capacity, or simply dissassemble them so they hold ZERO rounds.
Most gunstores will not sell you high capacity magazines because they have been "demonized" in NJ, and nobody wants to be the one to get blamed when some psycho buys a 20 round mag and goes on a shooting spree, then claims "I got it from Billy Bob's Gun & Pawn!" I've argued this scenario many times, but most people will not ship high cap mags to NJ. If you buy a Springfield Armory M1A in NJ though, the mags come blocked with a piece of wood. I'm not sure about other manufacturers... So while you live in NJ forget about anyone helping stand up for your rights. Once contraband, always contraband in NJ. Some morons (cops included) in this state think it's illegal to own hollowpoint bullets here. Hollowpoint bullets are illegal to be used in the commission of a crime in NJ (ooh, I bet that scares the bad guys from using a gun).
A LEO can own hi-cap magazines by getting a signed letterhead from his department, and the mags are supposed to be used in a law enforcement capacity. That means mags for your on or off duty gun, or whatever you use. The mags do not have to be owned by the department. If the department had to buy them, you would never see any. We buy them ourselves, and they belong to us, and can only be sold to other LEO's.
I highly doubt some law enforcement officer is trying to entrap you by buying two 20rd AR mags. With all the stuff you can buy off the net, I think a Beta-C mag would look "more evil" if he was trying to bust you... though if you ever read the newspapers, they call an SKS an AK-47, so it doesn't matter what you get busted with. The media will make sure it looks like you were a wacko gun-nut survivalist type.
You can also send him a refund, but I think that sucks. If he can legally own them, then sell them to him. If you lived in a state where you can legally own stuff, but everyone refuses to sell it to you, you would be pissed. I get tired of arguing with people about the legalities of what we can own and what we can't here. I just have my hi-caps shipped to another state or go out of state to buy them. As long as they're blocked/dissassembled when I bring them into the People's Republic of New Jersey, I'm legal.