In short, yes you can buy gun magazines online in NJ. Here is a more detailed answer:
You can purchase gun magazines online from several different retailers. However, there are some restrictions on purchasing ammunition and firearms in the state of New Jersey. For instance, you must be 21 years of age or older to purchase a handgun from a federally licensed firearms dealer. You must also have a valid Firearms Purchaser Identification Card (FPIC) to purchase ammunition or a firearm in the state of New Jersey.
Some laws regulate how many rounds a magazine can hold. For example, in September 2018, a law was enacted that limits magazine capacity to 10 rounds for handguns and 15 rounds for long guns. This law was put into effect to make it more difficult for would-be mass shooters to cause large-scale damage and casualty.
If you want to purchase a gun magazine online, be sure to do your research and only purchase from a reputable retailer. We also recommend checking the laws in your state or municipality to ensure that you are following all applicable laws and regulations.