Prepayment of potential Cap. Gains tax..in essense a state escrow not a tax.There is no Exit Tax. NJ requires the withholding for when people move out of state because they were not filing the income tax return that was due when they moved. If they file their return they just enter that amount as part of the withholding.
This type of withholding originated because out of state owners of land were selling to companies like Wawa and never paid NJ any tax that was due.