Organize your old "National Geographic" magazines. Separate all damaged, moldy or torn issues from the quality ones.
Compile a list of your issues and the condition of each one. eBay Reviews offers tips on assessing a magazine's condition.
Check out the "Collector's Corner" and "Collector's Forum" on the "National Geographic" website. The Collector's Corner lists dealers who might be interested in your magazines. You can also indicate in the Collector's Forum the issues you're selling.
Contact dealers from the Collector's Corner. Let them know what issues you're selling. If they aren't interested, ask for recommendations of other potential buyers.
Contact rare book dealers in your area to see if they deal with "National Geographic" magazines.
Photograph the magazine covers if you want to sell through an online marketplace such as eBay. List the item with a description and photo.