no sales to 18 or younger , i.d. mandatory.
See the applicable provision of the Criminal Code of NJ:
Weapons. Any person who manufactures, causes to be manufactured, transports, ships, sells or disposes of any weapon, including gravity knives, switchblade knives, ballistic knives, daggers, dirks, stilettos, billies, blackjacks, metal knuckles, sandclubs, slingshots, cesti or similar leather bands studded with metal filings, or, except as otherwise provided in subsection i. of this section, in the case of firearms if he is not licensed or registered to do so as provided in chapter 58, is guilty of a crime of the fourth degree. Any person who manufactures, causes to be manufactured, transports, ships, sells or disposes of any weapon or other device which projects, releases or emits tear gas or other substances intended to produce temporary physical discomfort or permanent injury through being vaporized or otherwise dispensed in the air, which is intended to be used for any purpose other than for authorized military or law enforcement purposes by duly authorized military or law enforcement personnel or the device is for the purpose of personal self-defense, is pocket-sized and contains not more than three-quarters of an ounce of chemical substance not ordinarily capable of lethal use or of inflicting serious bodily injury, or other than to be used by any person permitted to possess such weapon or device under the provisions of subsection d. of N.J.S.2C:39-5, which is intended for use by financial and other business institutions as part of an integrated security system, placed at fixed locations, for the protection of money and property, by the duly authorized personnel of those institutions, is guilty of a crime of the fourth degree.
My advice is to ask your local police before you seek to sell a knife at a flea auction or elsewhere.
The foregoing is not intended to be legal advice upon which you may rely as I have not been retained for this purpose.