Personally I would have just called Buds and worked it out with them. They probably would have been willing to swap magazines for you.
The straw purchase provision was originally meant to stop someone who can't legally buy a firearm from getting a 3rd person to do it for them. The problem is the wording of the law. If you gave your friend money to buy it for you, then technically it would be a straw purchase. If he bought it with his own money and then sold it to you, that would NOT be a straw purchase.
It comes down to semantics really. But chances are pretty good that no one in law enforcement would give a damn if you're going through an FFL and doing the NICS check etc.
Then again this is NJ so god only knows what the bonehead law enforcement would do. They like making examples.