A new collector would ask the same questions I've been asking and would probably want to feel reassured of exactly what they are purchasing.
A complete inventory matching supplements to monthly issues and knowledge of the first 18 volumes as reprints or originals would assist you (very time consuming).
There is a dealer directory at the top left margin of the home page of this web site. Perhaps one of them would make you an offer.
Because your set is bound completely in the airplane globe leather binding (a very impressive collection), if it were complete, even with the first 18 volumes as "rebound" 1964 reprints, I would consider a value, at a minimum of $3000.00.
But this is "only if" all the above is correct.
You may try listing it at this price with a best offer option for a few months (the market is extremely slow at this time for NGM - bound or otherwise) knowing a minimum price you will accept before reducing your asking price/best price offerings.
Note, as I stated in my guideline, dealers will offer far less than you can get on eBay, but the disposition of your collection will be immediate.