Can I realtor/sales person use the commission to pay closing costs?

I don't know the relevant rules if any that might apply to this under the NJAR, but there is no legal barrier to your reducing or eliminating your commission in return for a seller concession to be used for closing costs. However, two things are important here.  First, if you are borrowing money to make the purchase, the details of this arrangement must be disclosed to the bank on the HUD-1 settlement statement, and the bank will have to approve the arrangement. Second, part of the commission you earn is typically paid to the broker under your business arrangement with the broker.  You need your broker's approval as well in that case, since your action will eliminate any payment the broker otherwise would have received from the transaction.