I just bought about 12 National Geographics, all from the 1950s. I plan to go back to buy the rest of them, as there are about 50 left! I did a bit of research and found that 1950s National Geographics are popular among artists and designers. I feel that the best design and illustrations were done from about mid 1940 to the early 1960's. Many of the ads for travel, airlines and ship travel were still trying to sell the "romantic" part of travel. Ship travel was popular, and traveling by airplanes was still new and exciting.
Photography took over and design became more bland. The magaiznes, while in mint condition, are only $2 each. However, if I remove the ads, they are worth about $3-7 individually, according the e-stores on e-bay. I plan to do exactly that, but I do not think they are huge sellers at the moment.
Here are a few of the ads from the magazines.
The Kenya, New York, etc series are part of a campaign for Bank America Cheques. There are quite a few of them, as I have found many of them for sale on e-bay.