I've inherited 8 National Geographic leatherbounds from my father, who has recently passed away. They are, as follows:
Vol. 67 (jan-june, 1935)
Vol. 68 (july-dec, 1935)
Vol. 69 (jan-june, 1936)
Vol. 70 (july-dec, 1936)
Vol. 71 (jan-june, 1937)
Vol. 72 (july-dec, 1937)
Vol. 73 (jan-june, 1938)
Vol. 74 (jan-june, 1938)
All of them have their associated maps sans 67-70, which don't seem to have flaps for one.
I've received mixed messages on what they're worth (from $15 to $45 for a pair). I've been researching, but I figure the best place to see if anyone would be interested would be here. If the tradeoff shows them to be worth selling, I'd be willing to part with them.
I have some photos here. The singular photos are of Vol. 70; if anyone wants closeups of the others, I'd be more than happy to take them!
I do not generally watch bound volume sales newer than 1917.
Having said this, I can tell you since August of 2012 the highest sales price (on eBay) for a 1913 complete year (just one volume for the year) was $159.95.
By comparison (to show how quickly the prices drop), the highest sales price for the complete year of 1917 (two volumes just like those you are trying to sell) was $50.
(Note: I keep track of NG magazine sales from 1888 through 1917 on eBay of loose issues AND the highest two sales of bound volumes. There are "many" sales of these bound volumes that don't come close to making my short list.)
Generally speaking, after 1920, complete years of bound volumes drop precipitously in value. Specifics that may increase their value are, leather binding versus cloth and complete issue binding, i.e. covers and ads, versus simply the numbered article pages. Of course, complete with supplements is assumed in this analysis.
I would suggest to you that the "outward" appearance of your volumes appears fair to good. Please note that there is nothing hard and fast about used book sales. You might just find that one person looking for exactly what you are selling.
Hope this helps - good luck!