I've found a number of old NG magazines, bulletins, journals ranging from the small NG School Bulletins circa 1965-67 to journals (1916, 1930 & 1966). How do I assess value for these and where is the market?
A good source of information would be John Carey.
He has the following web site: http://www.jbcarey.net/ and I'm certain he would be happy to share with you what he knows.
He is also a member of this forum, but I'm not certain how often he comes to it.
I am not familiar with the market for any other NGS items other than the Magazines and the Indices, both Cumulative and Annual, but like all things NGS, there most likely is one (a market).
Good luck, and perhaps you'll want to come back and share the information with us.