Silver age Marvel comics are in huge demand. However, if you're looking to invest for the long term, these 6 categories of Silver Age Marvel comics have historically and consistently outperformed the rest of the market and are the most sought after by comic buyers.
1. First editions / Number 1 issues -- Collectors like to buy first issues of a new series. This includes Spidey, Xmen and Fantastic Four. It most cases the #1 issues remains the most valuable comic of a set. If that first issues is also the first appearance or origin of a character or team it is always sought after.
2. Cross-Overs/Key Issues - When characters appear in another character's book (Daredevil crossing over into Amazing SpiderMan), these cross-overs are always popular with collectors. Battle issues between cross-over characters (Hulk vs. Thing,) have a high demand, especially in older comics. Issues which affect major storylines and/or deal with the death of a major character (death of Gwen Stacey) are always highly sought after.
3. Reintroduction of Characters - The re-emergence of older characters from an earlier time (Capt. America - Avengers #4) or a restructuring of present ones (when Marvel Comics re-introduced Ghost Rider) is in consistent demand by collectors. Consequently, the first series of Ghost Rider and subsequent appearances rose dramatically in price.
4. Age - Generally speaking, Golden Age (l938-l945) comics are still most valuable than Silver Age (l956-l969). But Silver Age books are more valuable than Post-Silver Age (l970-present) books and everything else. At the present time Silver Age comics are in extremely high demand due to the introduction of Marvel Comics characters in this era and the high cost of purchasing Golden Age material. On the whole, new comics bought today will never be as valuable as comics from the Silver Age earlier eras due to rarity.
5. Hot Artists & Writers - Popular artists and writers (Jim Steranko) can create extremely high demand for even second-tier Marvel comic books (like Agent of Shield).. Many collectors will follow these artists and writers from title to title and wait with anticipation for their next move or project. Demand is usually increased dramatically when work is done for a major character. For example Jack Kirby's work on just about anything remains in high demand.
6. Movie hype - Movies featuring SpiderMan, Xmen, Silver Surfer, Fantastic Four, Hulk, and Iron Man have all created a buying frenzy for their comics from the Silver Age. More and more collectors jump on the bandwagon and buy up several copies so as not to miss out on a potentially valuable investment. This in turn brings the price up even more and buying frenzy increases again. This kind of demand will increase the price of books very fast. However, keep in mind that price guides are based on retail or dealer sales, not on personal sales, therefore the true value of a comic will only emerge when collectors attempt to resell them.