Then it's not an exit tax, it's a real estate sales tax. How do they know it's the intentions of the seller to leave the state?
Honestly, unless this is somehow charged like a "Departure Tax" in a third world shithole (the only places that have such totalitarian bull shit taxes), there is no mechanism to collect other than voluntarily. It would have to be charged after the fact to be valid. It would be like a penalty for not filing NJ State Taxes, in which case you're already gone... Again, I don't understand how this could be enforceable without enforceable borders on the state.