Are real estate commissions on the sale of the principle resident of the decedent deductable on a NJ inheritance return?

The NJ inheritance tax is based on the market value of the property at the time of death. You can't reduce its FMV because of expenses incurred after death to maintain, or later sell, the property.

However, selling expenses will reduce the amount of capital gains on the sale - so in this sense it is deductible from the heir's income tax (assuming there's a gain, and also that the sale of the property is not otherwise exempt from capital gains tax).