After a somewhat long perusal of the web to find the right New Jersey regulations regarding possession of nongame and exotic species of wildlife, I can not see anything that prohibits, in general, the keeping of native species. In fact certain native species are mentioned as exempt from the permit requirement that burdens keepers of other herps such as any species of Elaphe, or of Bombina bombina (the Fire Bellied Toad - but apparently only the Asian ones), all Geckos except the Tokay Gecko (probably because of its wonderful disposition) and Green Iguanas (can you imagine that, Green Iguanas one of the biggest pains in the butt in the industry/hobby and dangerous as adult males and they do not require a permit while a Leopard gecko requires one).
Among the exempted species are Painted Turtles, Snapping Turtles, Ribbon Snakes, and Garter Snakes, all of which, I believe, are native to New Jersey. I did not find the Diamondback Terrapin specifically mentioned, but I sort of looked here and there after finally finding the right pages.
By the way I was wondering what makes you think they are poached? Was it the price. That too makes me think they are probably not captive bred, but as to the other species mentioned you can possess them without them being captive bred. Maybe they were taken from the wild, and maybe that was illegal; but maybe it was done in a law abiding manner. Isn't it also possible that the person had a permit to catch and possess them legally?
If you search their site further maybe you will find some info on Diamondback Terrapins one way or the other. Being that they have long been considered to be a food species (for human consumption) they are possibly legal to keep, but that is just my guess. Here is the web address so you can have some fun checking for yourself:
NJ Regulations reference nongame & exotic species and here is the link to the main page:
NJ Division of Fish & Wildlife
Here is a contact link URL=http://www.nj.gov/dep/fgw/contact.htm]NJ Division of Fish & Wildlife Contact us[/url] So if you actually find out you have discovered a poacher or other type of violator, you have the means to turn them in to authorities. Happy hunting...
Best regards,
Glenn B