A private sale will net you the most money, if you can find a local buyer.
For that purpose, you should put up a Want to Sell (WTS) ad here on NJGF after becoming a Premium Member (even if it's only for 1 month at a time)
Setting up a sale on Gunbroker incurs fees (both FFL and Gunbroker charges) that you would not incur with a private local sale.
On the other hand, setting up an Gunbroker auction may, in fact, bring you more net money than a fixed price sale (private or Gunbroker) if you are selling a particularly desirable gun.
Trading it in or selling it to a local FFL means you are selling it at wholesale, as the FFL needs to mark it up to resell it at retail.
My humble opinion:
Pay the Premium Member fee and put up a WTS ad here and keep it in our NJGF 'family'. If it fails to sell here, you can always choose from the alternatives I explained.