Does anyone know how to go about selling old National Geographics?

There isn't much of a market for old Nat'l Geographics. You can get the whole magazine now on CDs, going back to the 1880s.

I would recommend putting an ad in Craigslist. It's free, so there's no downside. I don't know whether it would be better to sell them individually or as a collection. Ebay would be okay too, but EBay is global and Craigslist is local, and shipping those mags is going to be more expensive than they're worth. If you sell them locally, the buyer can just come and get them.

As horrible as this sounds, it might be better to cut out the full page ads and frame them and sell them that way. Some ads from the 1910s and 1920s were very beautiful. National Geographic put all their ads at the end of the magazine in the old days, and when libraries bound them into volumes they'd usually tear out the ads and throw them away. But today they are some most interesting things in the magazines.

Ebay would be your best bet.