Yes, it is illegal to sell striped bass in NJ.
23:5-45.3. Sale of striped bass prohibited
No person shall sell, barter, possess for sale or barter, or offer for sale or barter any striped bass, whether caught within the jurisdictional limits of this State or otherwise, provided that this section shall not prohibit the shipment or transportation of striped bass from another state or country into the State which is destined for shipment or transportation out of the State or prohibit the sale of commercially raised hybrid striped bass.
L.1991,c.43,s.2.
Are you sure they were not hybrid striped bass that were farm raised? I see these a lot in the food stores. They are technically hybrid, but they selectively breed the "black" bass out of them so they look and taste just like pure strain, wild caught stripers. However, they are a cross and are sterile and cannot reproduce themselves.