If I get a medical marijuana card from NJ can I use it to purchase medical marijuana in NYC

NO

No, here is a list of states that do.. https://www.google.com/amp/s/www.marijuanabreak.com/out-of-state-medical-marijuana-cards/amp Just google, states that will accept out of state medical marijuana card.

No way!

Nope

As of now no but other states do allow cross state buying.

For us