1. Condition: Magazines in excellent condition, with minimal wear and tear, will generally command a higher price than those with significant damage, such as torn pages or missing covers. Factors like water damage, mold, or extensive fading can also affect the value.
2. Completeness: A complete collection, with all issues present from 1913 to 2000, is more valuable than an incomplete set. Missing issues or gaps in the collection will reduce the overall value.
3. Leather Binding: Genuine leather bindings can increase the value of the collection, especially if the bindings are well-crafted and in good condition.
4. Special Issues: The presence of special editions, supplements, or commemorative issues can enhance the value of the collection. These special issues are often sought after by collectors and can add to the overall rarity and desirability of the set.
5. Original Advertisements: If the magazines include original advertisements from the early 20th century, these can be considered valuable additions to the collection. Many collectors look for issues with vintage ads, which can provide a glimpse into past eras and consumer culture.
6. Provenance: If the collection has a known history or provenance, such as being owned by a notable individual or institution, it can add value and make it more appealing to potential buyers.
With all these factors in mind, a complete leather-bound collection of National Geographic magazines dating from 1913 to 2000, in excellent condition, and with special issues and original advertisements, could potentially fetch several thousand dollars on the rare book or collector's market. It's advisable to consult with experts in the field to obtain a more accurate and up-to-date estimate of the value.